Grapevine rupestris feathery vein virus is not related to Fanleaf virus, and is not mechanically transmissible. It is closely related to Grapevine fleck virus.
In reply to your quest: to our (limited) experience dating several years ago - Grapevine rupestris vein feathering virus (correct virus name - not as reported in your message) population is extremely variable which hampers reliable detection.
Please take a look on Pasquale's recent paper that perfectly summarizes difficulties associated with GRVFV detection (as a side results of their study) - reporting several primer sets (from our lab, Adib's lab and from Bari).
Hope it was useful... Sorry,for the brevity of the message - trying to submit a proposal on time (deadline tomorrow)!!!