Sevin. They could probably use it every week but it will probably take a couple weeks before their numbers get back up to being a problem again.

I forget what the PHI is but it's short.

If you don't want to use chemicals, then dunk the beetles in a pail of soapy water. Just hold a container with a few inches of the soapy water under the vine and shake it or push the beetles off. They have an instinct to dive straight down when disturbed. For me the bucket of water works as well as most insecticides since the beetles only seem to die when you spray them and there isn't much residual effect from the chemicals that I've tried. And sevin is bad for honey bees (not that they'll be on your grapes, but I've got roses and wouldn't dare use sevin on them).

I don't think you'd manage with a bucket under my vines. That works OK on new vines though.

The Sevin seems to kill them most of the week as they eat. I've found new bodies laying around days after spraying.
One spray knocked me down from having swarms to having so few that squishing was an option. A thumbnail to the thorax seems to do them in.

The organic approach is to lose the battle and win the war with biological warfare, milky spore. It won't fix the problem this year but is supposed to take care of it long into the future.