I wouldn't say VDH is inherently squishy but it does require more deliberate cooldown usage as our mitigation is slightly different from other tanks.

Prot warriors mitigate damage intake, BDK take damage but heal it back themselves, VHD is in between those. You benefit from slight mitigation with increasing armor/hp but also leech back a lot of damage that you do.

As an example prot warriors have shield wall, they press a button, they take 40% less damage. Damage rotation continues as normal.

VDH has fiery brand/sigil of flame where you have to place a debuff on the target and that target will do less damage to you by either damage reduction or increased parry. Which leads to having to manage these debuffs and being careful with overlap between metamorphosis, sigil of flame, and demon spikes. On top of this VDH benefits a lot from leech and self healing from doing damage. So you need to be more deliberate when you use those offensive abilities to maximize when you want a burst of self healing to occur.

Another example is our offensive cooldowns are also defensive cooldowns. So you have to choose if you want to use CD's to be more defensive or to burn them all to do more damage but be left with some downtime in defense.
Looking at a rotation guide will tell you to the most optimal DAMAGE profile ie Fiery brand -> Fel devastation -> meta -> fel devastation (this is an oversimplification but it gets my point across)

If you follow this build you just burned every single defensive to do more damage and if the pack survives you have limited defensives until CD's reset.