It‘s possible to use VR headsets with the Steam Deck. It‘s just far too weak for a pleasant experience.
It‘s worth noting though, that, according to this interview (37:30), Valve is probably working on a new VR HMD which will make use of the things they‘ve learned from developing the Deck. So I‘d guess there‘s a standalone headset coming from them at some point in the future.
This is very speculative, but along the things they’ve learned for VR they‘re mentioning APUs and wireless streaming. This might hint towards a standalone device which can also be connected to a PC.