As already mentioned, the vComputer view already uses the Inv_AeX_AC_Primary_User table to display its User column value, so that requirement is already taken care of.
In order to only display a certain number of characters found within a column, you need to use either the LEFT or SUBSTRING commands.
I believe that the following query suits your needs:
SELECT
[Name] AS Computer,[Domain],LEFT ([User],1) AS [User],[IP Address]
FROM vComputer