I think Mata behind Rooney is better than Rooney behind RVP but nice options to have. Difficult to get all 3 on the pitch and have a balanced side though, especially against better opposition. If I was United I wouldn't buy a striker unless it was to bring in a 3rd choice level one to replace say Hernandez. A striker good enough to dislodge Rooney and/or RVP is going to cost 30 million plus and simply they have to spent the money elsewhere.
Also with VG now appointed, RVP is almost certainly going to carry on as the main striker. What he does at number 10 and out wide is anyone's guess. Looking at their first team, you could easily make a case for over half of it being replaced. That is a massive job to undertake, even with money as Spurs showed.
To compete for the title again and not allowing for some massive changes in form from the likes of Young, Fellaini and Nani, they arguably need a whole new back 4, 2 central midfielders and arguably a winger.