Too many buds allowed to grow on a first year cutting will create canes that are weak. Only allow one bud to grow, and train it up. Don't let canes sprout from it. It's hard work, pinching all those excess canes, but you will get a sturdy trunk for your efforts. Once the main cane reaches the height you wish, then pinch the top and allow side canes or cordons to grow. As the canopy grows, it will feed the roots and they will strengthen the trunk and canes.

Don't allow the canes to fruit. Pinch all fruit clusters until the 3rd or 4th year. The vine needs to establish a good root system before it can bear good fruit.

As to your question you can bud onto a rooted cutting, but as long as your cutting has buds, just eliminate all but 2, and once they sprout, pinch the weaker one.

I did let just one bud grow. As I learned more , I decided I would like more of a different vintage, and wanted to convert some existing rootstock over to the new cultivar. This was more of a grafting question than growth question. Our season is super short here, with very limited options. My new growth was fine , just small diameter.

Never grafted a grape but am going to try it this year. Going to use a wild grape rootstock and use a method similar to this video http://www.youtube.com/watch?v=STHWxaR8vW8