I was wondering if there is another RPC that would allow me to get the vsize easily without building the transaction first.
There is not.
However, you can use
fundrawtransaction which will do size estimation to apply the fees for you, and add a change output if necessary, and add additional inputs if necessary. It will use the inputs that you have already specified in the provided transaction.
Furthermore, as I have said in previous posts, you can do all of this with the
send RPC. I don't understand why you are insisting on doing everything by hand the hard (and deprecated) way when everything you want to do is already built in.