I’d add that considering Vegeta’s point of view; he’s calling him Kakarot as a sign of messed up respect. As he’s really racist and prideful, and views all the other races weaker, him using the name Goku would mean also lowering himself to the level of “lower races” as he’d have to admit sayans can be on the same level as them. Him using Goku more and more would be in effect a sign of him slowly becoming less racist

edit// fixed words