Me too...which is why you don't do it this way. With the AP engaged and presumably in ALT mode, simply hit SBY ...or deselect the ALT mode.....or hit ALTSEL (all 3 cancel the ALT hold mode) and then use the pitch wheel to dial in the VVI you desire....WHILE the AP is still engaged. When you achieve the rate of climb/descent you desire via the pitch wheel, then simply hit the VS mode button to hold that value. Easy peasy....sure, not quite as easy as modern copilots for sure, but neither terribly bothersome either.
TK