Can a lender in second lien position (heloc) foreclose on a mortgage when the lender in first lien position has not foreclosed? If so, do I have to vacate the property if the first lien holder has not foreclosed?

Foreclosure law varies from state to state. It may be a part of the lender's collection (harassment) technique. It may be something the lender does before paying off, and taking an assignment of, your first mortgage; then foreclosing on the first mortgage. As you know, an unpaid note just gets bigger and bigger, so try to find a way to stay current. I don't believe the second lien holder can force you out because, as you suggest, the first lien holder has a superior position.

You also need to read your contract with the holder of the second.