I don't think I've texted during an actual vacation (like when T or MC was out of town), but I did text my marriage counselor recently over a holiday weekend (like a 3-day weekend). And he wrote back. The next session, I said how I'd really wanted to call him because I was really struggling, but didn't want to bother him on a holiday weekend. And he said it would have been OK to call.

I feel like since your T specifically said it was OK to text, then it's fine. They wouldn't offer it if they weren't OK with it. I mean, don't text them every hour just to be like, "Hey, how's it going? I'm bored. Weather is kinda hot today" or something like that. But if you're in a bad place or just want to make sure T is still there, then I'd say go for it.

Oh wait, actually I did text MC once when he was out of town, but I didn't know he was away. I didn't hear back and followed up a couple days later (was concerned he was annoyed with me), and he said everything was fine, that he'd just been out of town and busy, so I was like, "Oh, sorry, didn't realize that!"