Question - Vampiric Touch - D&D5e

When I cast Vampiric Touch off of my spell sheet, is there a way for the spellblock to calculate the health I should get back and display it as part of the cast?

Unfortunately, probably not in a useful way; there is no way for an inline roll [within a sheet-provided Roll Template] to make such a calculation based off another [inline] roll while accessibly displaying both rolls in full. The API can, however, accomplish this. /r floor([[3d6]]/2) would display the damage (within an inline roll) and calculate the hit points regained, yet—as it is a (non-inline) roll—it cannot really be used in conjunction with the (Roll Template) output of a sheet button.