They're totally different. Boost isn't a great running shoe. There's almost too much bounce. The vapormax are more firmly tuned, so they really respond when you push them beyond normal walking. I find the knit of the vapormax to be the best fk upper that Nike has made and somewhere in between the stiffer 1.0 boost knit and loose 3.0 boost knit (don't own any 2.0).
Boost would probably be more a lifestyle oriented shoe. Vmax a more performance oriented shoe.
There is however a secret weapon the vapormax has. It's stupid light and flexible. I've never felt anything quite like it. I'm pretty confident it's lighter than a Flyknit racer. Im sure that will make it very easy to wear for extended periods of time. (I've only spent limited time w it.)
personally, I prefer the vapormax in limited wearing. Mostly cause of the weight and overall aesthetic (which is super subjective). As much as I'm a Nike guy, I've got a pretty filthy boost collection that I don't plan on retiring BUT these vapormaxes are the first new air model in a long time that I can honestly say I'm excited about - got 3 pairs and another one on the way tomorrow. Happy air max day.