It depends on your objective. If you are thinking you will use this for healthcare expenses then leaving it as cash may be best. If you are thinking you may use this for healthcare expenses or otherwise want it conservatively invested then VTINX is a great choice, well diversified and mostly bonds. If you want to invest it, and the rest of your 401k et al is in S&P500 and US Total Market funds, then the international fund is a good choice. If you are following a broader asset allocation, then invest according to that.
FYI you can port the HSA to anywhere, so you don't need to limit yourself to those funds. Fidelity is probably the best choice if you choose to do this. I have not moved my HSA because it is too much work, and I invest mine in Vanguard Midcap Index.