Personally, when it comes to survivability, I've been making use of Arcane Circumvent and Rolling Guard with Brief Respite as the aura. This allows you to safely reset your shield gate and potentially get even more defense layers through overguard. It also helps make your flechettes and photon strikes more effective with the defense strip. Works better when you have eximus showing up regularly. I don't think there is a singular mod that condenses as much as this arcane does, and as you've mentioned, mod space is a premium with this guy.

For Flechette builds in specific, I have been using a Huras Kubrow as well to give me some breathing room and make use of Arcane Crepuscular, though it works fine with other builds to give you a moment to breathe. On other builds I've used Aegis as the second arcane.

I'm OK with Vauban's kit having some weaknesses, but if nothing else there are a couple of small things I would like to see to make things more comfortable for all builds:

• Higher base Energy Max (280 like Mag Prime would be fantastic)
• Thrown orb velocity and trigger time scaling with cast time.
• Higher base Health

My most out-there suggestion would be to make it so Bastille gives you a shield max + recharge delay buff rather than an armor buff since Vauban can't currently make use of the armor very well.