There is a movement in the major HSA admins to offer low cost investment options which typically means Vanguard. They are targeting what they call savers - those that seek to max contributions, limit withdrawals, and want low cost index options.
Health Savings Administrators was an early leader of this movement and still one of the few that have a first dollar investment option - i.e. You don't have to get up to say $1k or $2,500 before you invest. Lately, HealthEquity, WageWorks, and UMB have gotten in on this idea. More will follow them.
So good news is you have options now and will have more in the next year or two.