You DO NOT need an IFR aircraft to fly VFR on top, otherwise why would the AIP allow it ? See
AIP ENR 1.1 17.2 VFR Nav. Requirements
To fly VFR on top does not necessarily mean you will have to climb/descend through cloud.
I have flown VFR on top on many occasions and have still remained in VMC during climb/descent.
This is usually possible on very long flights (several hours) in which you might cover several hundred miles. Most pilots would be well aware how the weather can change over such a long flight. You can depart/arrive in VMC, however be VFR on top maybe part of the way enroute. You would always check the weather forecasts/reports before you depart and enroute to ensure that VFR climb/descent is still possible.
For example - you might depart early morning in VMC, fly inland and be VFR on top of a low layer of fog/stratus, and as the day goes on the low level cloud disappears. However this is not the only scenario when it can happen.
I did it many times, and always VFR