LOL -- yes, you and that casket...
Would you have him treated like Lenin or
is it Stalin who's entombed above ground?
Lenin is above ground, though his glass case is kept airtight. There are also Catholic saints with intact bodies above ground.
An embalmed body in a vault below ground will retain its appearance for a very long time, though the skin will darken.
Actually, a body in a mausoleum decays much faster, due to the exposure to air.