I think it's just a question of money and the amount of time and work you want to put into finding a bike. If you can find the right vintage frame in good condition, in your size. I see no disadvantage to going that route. And you will save a lot of money. Might not even be a question of time as most custom builders are so backlogged, it may take just as long to get a new frame delivered as tracking down the right vintage one. FWIW, If I were looking at new I would look at a Mercian before I bought a Rivendell.