Rodrygo and Vinicious are different situations. Rodrygo isn't considered in the team by Xabi for starting, he is considered as the best bench player, like to replace anyone, he can replace anyone at attacking side, so he can be bench for like 4 different players, so he will get ton of minutes without a shred of doubt, more than some starters even I am sure depending on injury and fatigue and what not.
However, Vinicious is still considered, he is still their star player, along with Mbappe. But the sad reality is, Mbappe is also a left winger normally, just because he plays awesome at striker, doesn't change the fact that he wants to play at left wing. If it was up to Mbappe, he would sell Vini, go back to left wing, and use the sale money from Vini to buy a striker.
Why vinicious is still considered as their star is basically because he has more playing time than Rodrigo, players improve their quality when given good amount of time, just as you said, Rodrigo stay more on the bench and for a player within his prime, that kind of situation reduces your quality. This is part of the reason Ivy said Rodrigo would have left Madrid at the end of the season, because obviously the quality of players he has to rival with wouldn't give him that opportunity of regular time of play. When you look at the Madrid attack, Jude Bellingham is always coming from the middle as an attacking midfielders, mbappe stand's on the top right in some cases as the finisher while Vinicious comes from the left, so these makes Rodrigo to be left on the bench most cases.