If these two players develop a good relationship, their game will become more dangerous. Vinicius Junior can create great balls from the wing position and supply the strikers. If Mbappe can finish those balls well, then Mbappe will score goals in every match and Vinicius Junior will be there to help. In fact, not everything in a team is perfect, for example, Real Madrid has bought three players who love to play in the same position, Vinicius Junior wants to play in the same position, Rodrygo wants to play in the same position, and Mbappe also plays well in the same position, but arranging an attacking line-up with these three has become a very challenging matter for Madrid. If Vinicius Junior is put on in a match, Rodrygo is put on the field instead of him, and when Vinicius Junior comes on the field, Rodrygo cannot come on the field. 
If one of these three players could do well in the right-sided wing position, Real Madrid's attacking line would be even stronger.