working with self leveling compound
It is roughly the consistency of soup, and will flow to any point not blocked. It goes down pinholes, etc.
But I am confused about your statement on the under floors. The subfloor is generally the bottom most layer, and originally installed when the house was built. Depending on what is over that, can make or break a new floor, especially tile. Vinyl is usually not an issue, but having a songy layer under ceramic tile will cause grout and/or tile breakage.