Believe it or not, the VA/SQ arrangement is *extremely* unusual. It started when SQ owned part of VA. It's a minor miracle that it came back recently.

It's a common misconception among people that FF miles/points are like currency and can be traded. As a general rule, there's no such option. When it is an option, they make the 'exchange rate' ridiculous and/or charge large fees to do so. There are rare exceptions like VA/SQ.

It is not possible between UA and VA, and the odds are it never will be. The idea is that you accrue miles/points in one program, if the airlines are partners. So I would suggest you accrue VA points on your UA flights, rather than have multiple programs. Of course, that won't help with UA miles upgrades.