If your vine has started to break new buds it does mean that no more pruning can be done. If you prune after the sap has started to flow it can be very difficult to stop it bleeding. There is a wine produced in Germany that is made form grapes that have been frosted, it gives a very special taste to the wine. We did live next door to the son of the wine grower who first found out about this after a very early frost in the autumn.