I didn't get the impression from the OP that the yearly limits were in play, just some technique to "fund" the yearly contributions with existing shares of VTSAX instead of using new or existing cash. I agree it is not very clear in the OP, but doubtful (?) someone posting here would not be aware of the limits.
I read it as OP wanting to let cash build up in an account (this cash coming from a job), and sell VTSAX in taxable and turn around and buy VTSAX in IRA. I have no idea why someone would do that, but that's how I read it. Actually, I guess in some narrow "want to shift some funds to cash, but don't want to be out of the market, but still fund IRA as a equity position" scenario I could see doing something like this. But there are possible tax considerations to be discussed. Likely possible to do it without an impact (with $300k in taxable there are likely some lots with losses, or carryover losses around...)