My theory is that against Cell, Trunks actually went LSSJ. Cell was simply just a far more advanced being and Vegeta couldn't understand how his son could be stronger than him and so they both "teamed up" on LSSJ Trunks and that's ultimately why he failed to take down the Perfect Cell early. Fathers..... let your children succeed.... or face dire, dire, dire consequences... You've been warned
I've actually never watched this full fight, as when it was aired I was 13 and would miss some episodes, so here we go
https://www.youtube.com/watch?v=Meyd0Uq4qpA