Los Blancos have be tracking French star Mbappe for the last 18 months and his current Paris Saint-Germain contract is now into its final year.
With no progress on new deal, the La Liga giants are confident of bringing Mbappe to Madrid, with PSG facing the prospect of losing him on a free transfer in 2021.
However, Carlo Ancelotti is facing a financial crisis in the Spanish capital this summer and he will need to raise funds before moving for Mbappe.
According to reports from the Daily Mirror, Carlo Ancelotti is open to selling Vincius to free up space for Mbappe, with United quoted £68.5m for the Brazilian international.
Vinicius has not expressed an interest in leaving Real Madrid ahead of the 2021/22 campaign and United will push for a loan to buy deal in order to bring their asking price down to below £50m.