YQ is a surcharge imposed and collected by the airline to its pax and kept by said airline.

A tax is a payment required by a government which all airlines must collect and pass on to such government. OTOH, when airline XX collects its own YQ, they get to keep it. So XX air shouldn't really collect YQ on behalf of air YY, because the latter will most likely not see a cent of it.

When it comes to US-based airlines, AFAIK they are not authorized to bill their own YQ separately, so they can't charge it as a separate item on their own award tix, at least if they are issued in the US. Therefore, a third party (VS in our case) shouldn't be able to collect YQ on behalf of CO/US. I think DL might be breaking this rule for its own award tix issued in Europe, probably because the no YQ requirement doesn't extend to those (not sure, though).