MS shall try to connect to its HPLMN first. But i agree that as soon as you give the possibility during the reselection, you can’t really know what is the MS going to do.
(sw implementation is not always perfect…)
But in your case, you don’t need reselection ! Sorry, I didn’t notice earlier, but if you’re OK with a drop, a network loss and then a selection, you don’t need to define any inter PLMN behaviour in your BSS.
In the SIM card, the other PLMN shall be in the list of authorized PLMNs though.
The MS will automaticaly select another PLMN (the vPLMN) if the HPLMN is not available. The problem is that sometimes, the MS doesn’t come back to the HPLMN, if the vPLMN is still available.