Reservoir Package in Groundwater Vistas
Reservoirs are special versions of Lakes in Vistas. So you add them by using BCs/Lake. You'll see a check-box on the dialog that says this is a reservoir. Then some fields will be grayed out and others will have different labels according to what the RES package needs as opposed to the lake package. RES package is turned on and cell-by-cell flow unit number set under model/modflow/more packages. The few options are set using model/modflow2000/options - Lake3/RES tab.