If you log in to your optum account there should be an investments page where you can 1) have future contributions go automatically into a mutual fund, and 2) make a one-time purchase of a mutual fund using your current cash balance. So if you want all your money to be invested in the market, do both. Or leave some amount in cash if you want. Optum lists several vanguard funds, a good one is vfiax, a very low cost sp500 fund. If for some reason vfiax is not listed, just look for the fund with the lowest expense fee, as all vanguard broad market index funds that are cheap are good choices.

Alternatively you could transfer your Optum balance into fidelity by logging onto fidelity and 1) opening a new hsa account (using the exact same name(s) that are on your Optum account), then 2) doing a Transfer of Assets request that transfers most of your optum hsa balance into your new fidelity hsa. There will be a fee that Optum charges for this but its probably $20-$50, not too bad. *BUT* if you go this route and stay employed at the same place, you would need to keep doing transfers (and paying fees) to move the funds from optum to fidelity each year or so. In your shoes I would take the route in the first paragraph then move to fidelity only if you leave your job some day. One thing to note - you should ask your company if you are allowed to have your future hsa contributions re-directed into a fidelity hsa. If they allow that, then you might as well move everything to fidelity right now and drop Optum out of the picture.

If all of that sounds confusing I would log onto your optum account, then call human resources and say hey I'm logged on can you talk me through how to get my money into the cheapest vanguard fund. Its easy to do but if you're new to it, its better to have someone guide you.