There are no hidden fees. It is indeed cheaper to buy and hold Vanguard ETFs on the Trading 212 platform than it is on Vanguard’s own platform. Vanguard fee is 0.15% platform fee capped at £375 per year. Trading 212 has no platform fee.
VUSA on both has the same 0.07% fee taken by Vanguard, and Vanguard pays 15% US withholding tax on dividends they receive, but you do not see these explicitly. So that amounts to about 0.35% annual loss compared to owning S&P 500 yourself. The Vanguard platform fee takes it to about 0.5% cost.
I expect Vanguard see themselves as needing to compete on fees with the likes of Hargreaves Lansdown and AJ Bell, and do not yet see Trading 212 as a high enough profile competitor to worry about. .
An advantage of the Vanguard platform is that you can buy a huge range of index funds, like the Lifestrategy range, and open a SIPP. These possibilities are not yet available on Trading 212.