Though I think your approach is good, I have to say, if I was married and found my husband posting all over a message board about his "progress in getting me to go vegan" and assuming she will "make the switch". I would feel like he was being pretty underhanded and sneaky about the whole thing. Yes, just my gut impression.
You say you haven't talked about it? Weird, if you talk about everything else. Why haven't you brought it up? Not in a confrontational way, but complimenting the things you notice that she already IS doing... "Honey thanks for being willing to drink soymilk too, it makes this all easier for me...".
I hope she also gets support from you in the possibility that she NEVER makes the switch. You sound like you're assuming a lot without ever having talked about it.