At this time of the season, I am often asked about an application of fungicide(s) to dormant vines. I totally understand that you want to do something before things get busy.
A dormant application of lime sulfur (10% in our study, or 1% of a newer product called Sulforix in a study done by Dr. Annemiek Schilder at Michigan State Univ.) should be effective against Phomopsis and/or Anthracnose. We also tested cupper, but it was not effective. However, the efficacy of the application is not strong enough to allow you to skip any preventative fungicide applications to be sprayed soon after bud break. (i.e., even with a dormant application, you still need to protect your shoots when they emerge.) With the corrosiveness of lime sulfur, I feel that it is not worse the money and time, plus, it will be another application of a fungicide, which we try to reduce. It is much effective to spray mancozeb or captan soon after bud break. Thus, I would recommend a dormant application of lime sulfur only if you have a serious issue of Phomopsis and/or anthracnose and you need an extra kick to your regular preventative application after bud break.
The dormant application is less likely very effective against downy or powdery because both are called polycyclic diseases with a rapid secondary cycle. Even if you knock down the initial inoculum, they can produce next round of spores rapidly especially under favorable conditions. Also, the winter survival structures of these pathogens are very tough, so, I don't think fungicide application is the best approach.