I have a small backyard vineyard of 50 vinifera (riesling/sangiovese/nebbiolo) . The climate here in SW Washington is ideal for powdery mildew, so air circulation between/through vines is something to think about. If the vines are 'crowded' then you will not have much airflow through the vineyard/between plants. The vines WILL grow into each other ... a lot. Any overlap between plants will make a dense canopy that will require pruning back or at least some leaf thinning in order to keep some air circulation.
My vines are spaced 6' apart and I am glad they are not closer. My space between rows is 6 feet, which gives me enough room to run a lawn mower through, in addition to have enough room for myself and a handsprayer.
I cane prune, so each vine will have a cane that will extend approximately 3' , one cane on each side....so they almost interesect with the adjacent vines. 6 foot spacing works well for me. 8 foot spacing would not be bad at all. However, 3 foot spacing would not leave much room for the new canes that you 'bend over' and the later canopy growth.
Certain vinifera do well with longer canes, more buds, which would require further spacing between plants. I keep relatively short canes and only 3-4 buds on each cane, focusing on fruit quality, and not so much quantity.
Spur pruning would be slightly different, as you might want to have longer cordons. And that would determine your plant -spacing.
This is the fun part about grape growing! You get to try all sorts of techniques. The amazing thing about wine grapes is that they grow like crazy and seem almost indestructable.
Good luck and have fun!