Suggest you cut it back until looks like a bareroot grape you would buy.

Or could retain just one sturdy cane, then train as suggested in this pub at the link below ..
In other words, handle that one cane as if the vine completed a year's growth.

Here is a link that might be useful: grow grapes at home

A single long cane, or a cane down to the 2nd bud? I just do not know how conservative I should be.

I agree with Jean, select one as if it were your central cordon (which it will be). Cut it at the height you plan on vineing out a quarter to half inch above that bud (depends on local recommendations). If you want a two tiered arrangement then cut at a bud around the height of the lower tier and let it grow up and two shoots grow out from around there this spring.

One exception would be a fan arrangement in which case you can use all of those vines. (see link for ideas. For a home owner I don't think there is a right or wrong or one way to do grapes. Yours is out in the open so will need some form of support. The doc is for commercial production outside your residence so some of what is said does not apply.)

Nice looking vine BTW!

Here is a link that might be useful: Training and Trellising Grapes