Both of your STORE statements reference the the field "VST_ID". However, VST_ID is not created in the LOAD statement. It may be simpler if you did not reference any fields in your STORE statement. e.g.
STORE VST_SUMM into TPP_SUM;
Also, it's conventional to use the .qvd extension on the STORE filename. e.g.
STORE VST_SUMM into TPP_SUM.qvd;
You're not getting the ID in your loop.
Why not just join?
OPV: LOAD*FROM [lib://Stage2 (uranus_qlikadmin)/OPV_Table.qvd](qvd);
IH_EMR_VST_ID as vstID,
Sum( SELF_PAYER ) as GUEST,
Sum( COMPANY_SHARE ) as PAYER
FROM [lib://Stage2 (uranus_qlikadmin)/C_REV.qvd](qvd)
Even if your loop did work, the resulting file would only contain the last ID as each would overwrite the previous.