If you're touring, you're not going to be moving very fast anyway. If you can find the secret sauce in a frameset and get the magic fit, the beefier tires and overall geometry of a vintage MTB conversion can make for a very rewarding ride.

The downsides of going vintage are that you'll need to completely strip and reassemble any bike you buy. You will almost certainly need to replace parts as well, which can quickly get you near new bike pricing.

I think the best way to answer this question is by asking another one. What would you gain by purchasing or building a dedicated touring bike, and what do you give up by not riding a hybrid or general-purpose road bike? If you're not carrying more than water, a couple changes of clothes, and a credit card, you don't really need the pack-mule capabilities of a touring-specific setup.