Herbicides pre and post for grapes
If the trunks are obviously woody, yes, but do a careful directed spray to avoid soaking them down. Also, make sure to remove any suckers before spraying.
My biggest concern right now would be buds developing along the stems/trunks. If you rub these off, then you have a nice open place on the trunk to absorb the herbicide. And you need to rub them off before you spray, and need to remove them anyway.
If you could do another form of weed control, or be very cautious with any directed spray, I think that best.