I'm in the same situation! I'm waiting on the formal offer letter for my TC but I got invited to
Dechert's VI. I was wondering about whether I should do it, since apparently it's quite standard, and I definitely will feel somehow kind of guilty if I don't do it as well. Like I'm still having those anxieties that my TC offer will fall through or something even though I've talked loads to partners and GR!
Personally, I don't think you should pressure yourself to do it if you're feeling like you're spread thin. There's some people out there that receive 3+ VS/TC offers and have to cancel on multiple firms. You're not ungrateful at all - considering that
Dechert doesn't take direct TC applications and their VS is the only way of them assessing potential candidates, if you're not serious about the TC with them and would rather another firm, there's absolutely no obligation on you to keep going through the process unless you wanted to or felt it would be some kind of big benefit to get the practice. Considering they definitely took their time with the invites as well compared to other firms, I wouldn't think you'll be the only one that's cancelling because they accepted an offer another firm made first.