I just hit level 38 and noticed I was at the 3k affinity cap for twin swords (wearing 3 wolf magatamas)

is it worth it to continue to invest into dexterity for the bow affinity (which is my secondary weapon, with 1366 affinity with a Chigaiya magatama) or should I start pumping points into vitality for more survivability?

Just stick it in dex if your only other plan was to put points into vit. For a dex player a bit of hp theoretically sounds nice, but you'll be at death's door within a hit even if you pump vit.(Assuming you didn't die already) Your pretty much in a kill or be killed situation from the start when you choose dex as a primary, heck same goes for everything other than a full vit build, in which case we can take 2 hits if we're lucky.

(Might be exaggerating a bit, but extra survivability will not mean much beyond being able to take possibly one extra normal mob hit and is kinda wasted if it could be used to kill your opponent in less time and thus sparing yourself the risk of getting hit in the first place. After all, you don't get hit by dead stuff, Right? *Unless it's that laggy shit, gl dodging that*)

Slowly swap out Wolf for Nekomata. Stop adding dex once you're at 3k without Affinity magatama.