Tax loss harvesting isn't a feature that you can differentiate yourself with.
I've implemented it several times over for the hedge funds I've worked at. Its a well understood tool that's been in use by funds for as far back as I remember.
A more realistic estimate is probably 6 people for 2 years. Plus increased support costs. Large financial companies are often ridiculously slow to roll things out, but as a customer you want risk aversion not bugs.
A good example of how estimates in software are usually off by large factors. Determining the best exit point to harvest a loss at, for example, would probably take more than a week. And that is just one small piece of the model.
I suppose you could do something like that within a sector - trading an Amazon for an Apple in tech, or a Shell for an ExxonMobil in oil - but that's betting on a lot of independent factors besides the performance of the commodity or sector as a whole.
In fact, I'm not even sure how Vanguard could do this in any manner that would be customer facing.
I can buy whatever companies ETF I want with my Vanguard account. Why would it be illegal?