FYI Vanguard cap @ £250,000, meaning the maximum fee per year is £375, this is the cheapest AFAIK.
You can open and put money into each type of ISA per year.
You've already opened a stocks and shares ISA this year, you can also open a cash isa, or an innovative finance isa or both.
Once opened you can deposit whatever in whichever you'd like, up to a combined total of £20,000 per tax year.
You should forget about trading 212, and simply keep your vanguard ISA, and invest regularly i.e. every month buy whatever funds you have chosen
If you really must trade, then in April you can turn your 212 into an ISA, while keeping your vanguard and deposit into both of them.
My prediction is you will stop that trading and end up with only your vanguard, quote this post when that happens.