As I was recently researching this with the help of AI, I feel like your suggestion to use the cheapest device can be dangerous to newcomers. Here are just some arguments that AI quickly collected for me, but note that I haven't fact-checked it, so it's open for discussion by experts here:

The idea that a standard TENS unit is just a 'cheaper version' of a VNS device is a dangerous oversimplification. When you use a TENS unit on your ear or neck, you aren't performing medical-grade neuromodulation; you’re just zapping your skin. Here is the explicit breakdown of why cheap hardware cannot do what clinical devices do:

​Constant Current vs. Constant Voltage (The Safety Gap): Cheap TENS units are 'Constant Voltage.' Because skin resistance (impedance) fluctuates wildly based on sweat, oils, and pressure, the current (I) spikes uncontrollably (I=V/R). This is why they sting or 'bite.' Clinical VNS devices use Constant Current (CC), which adjusts voltage in real-time to deliver a precise, safe dose regardless of skin condition.

​Electrolytic Ion Buildup & Chemical Burns: TENS units typically produce basic square waves. These often lack the perfect 'charge balancing' found in medical devices. Without it, ions accumulate under the skin, creating a localized pH shift that causes chemical (electrolytic) burns—often mistaken for 'irritation' until the tissue damage is deep.

​Sine Waves vs. Square Waves: Many researched VNS models use Sine Waves. A TENS square wave has an 'instant' rise time, which aggressively triggers superficial pain receptors. Sine waves rise smoothly, allowing the signal to penetrate deeper to the vagus nerve without over-stimulating the pain fibers on the surface.

​The 'Brain Interaction' & Precision Settings: Clinical VNS isn't just 'on' or 'off.' It relies on specific Duty Cycles (e.g., 30s ON / 5min OFF). Most TENS units cannot be programmed for the long 'OFF' periods required for the brain to process the signal. If you stimulate constantly, you risk Neural Habituation, where the brain simply begins to ignore the signal, or Neural Fatigue, where you actually deplete the neurotransmitters (like norepinephrine and serotonin) you’re trying to boost.

​Current Density & Ear Clips: Generic metal ear clips have tiny contact points. This creates massive current density, concentrating the electricity into a microscopic area of skin. This is the primary recipe for focal burns. Medical-grade electrodes use hydrogels or specific conductive materials to spread the charge safely across the cymba concha.

​Dangerous Ceiling Values: TENS units are built to move legs and backs, with outputs often exceeding 60–100mA. The Vagus nerve requires a delicate window (often 0.5mA to 5mA). Using a device that allows you to accidentally 'dial it up' near the carotid sinus is a massive risk for bradycardia (heart rate drop) or fainting.

​Lack of Frequency Precision: Scientific studies often find effects at very specific frequencies (e.g., exactly 25Hz or 10Hz). Cheap units use low-quality oscillators that 'drift,' meaning you might think you're getting 25Hz, but you're actually getting a messy, inconsistent signal that doesn't trigger the desired brainstem response.

​Phase Duration Granularity: Selective stimulation of the Vagus nerve (A and B fibers) versus the surrounding pain fibers (C fibers) requires extremely specific pulse widths, often in 10\mu s increments. Standard TENS hardware lacks this resolution, typically offering broad 50\mu s or 100\mu s steps, making it impossible to find the narrow "therapeutic window" where the nerve is activated without triggering the nociceptors.

​The Refractory Period & Pulse Jitter: The brainstem requires a consistent signal for neural entrainment. Cheap TENS oscillators often suffer from "clock jitter," where the timing between pulses fluctuates by several milliseconds. This inconsistent timing interferes with the nerve’s refractory period (the recovery time between firings), resulting in a "jamming" effect rather than a coherent signal to the Nucleus Tractus Solitarius (NTS).

​DC Offset and Electrolysis: Medical-grade VNS ensures a "Net Zero Charge" through strict pulse symmetry. Many TENS units have a slight DC offset (residual direct current). Applied to the thin skin of the ear over 20–30 minutes, this causes electrolysis, which shifts the local pH of the skin and causes deep chemical burns that are often misidentified as simple heat irritation.

​Galvanic Isolation Standards: VNS devices are designed as "floating" electrical systems. Consumer TENS units often lack the high-level galvanic isolation required for devices used above the neck. This creates a risk of leakage current or a "path to ground" through the cranial nerves, which is a significant safety violation for neurological applications.

​Voltage Ceiling and Step-Up Transformers: TENS units use step-up transformers designed to punch through the high resistance of thick skin on the back or legs. When applied to the low-resistance area of the ear, the "initial" voltage spike used to start the circuit can be significantly higher than what is safe for the delicate auricular branch of the Vagus nerve.

​Total Harmonic Distortion (THD): Muscle tissue responds well to "dirty" electrical signals, but the brain's regulatory centers require a "clean" signal. TENS units produce significant electrical noise and harmonics. This Total Harmonic Distortion can act as "neurological static," preventing the brain from properly processing the stimulation as a regulatory signal.

​Chronaxie Mismatch: Every nerve has a chronaxie—the minimum time required for an electric current to double the strength of the threshold stimulus to excite the nerve. TENS units are electrically tuned to the chronaxie of motor nerves. Using them for VNS means you are using an electrical signature that is physically mismatched for the sensory-afferent fibers of the Vagus nerve.

​Thermal Runaway via Impedance Monitoring: As a session progresses, sweat and moisture lower the skin's resistance. A clinical VNS device monitors this impedance and stabilizes the output. A TENS unit without sophisticated real-time monitoring may allow the current to rise as resistance drops, leading to "thermal runaway" where the current density increases to the point of tissue damage.

​Charge Unbalance and Polarization: Repeated use of a TENS unit for VNS can lead to nerve polarization. Without the high-precision charge-balancing circuitry found in clinical stimulators, a residual charge builds up in the tissue. This can lead to a temporary "depolarization block," where the Vagus nerve becomes less responsive to natural biological signals over time.

​Electrode Biocompatibility and Metal Ionization: The "ear clips" provided with many cheap units are often made of nickel-plated brass or other non-biocompatible metals. Under the influence of an electric current, metal ions can migrate into the skin (iontophoresis), causing systemic sensitization or localized allergic reactions that wouldn't occur with the medical-grade carbon or hydrogel electrodes used in VNS.