Following a topic on a forum I modded my VP1 for slower sweeps of over one minute. Someone came up with the instruction where you put a large resistor between the center tap of the rate potentiometer and ground.
I did however not obtain a satisfactory result with the resistor values suggested by the other poesters. I found that for my purposes a value of 680k was best. So maybe you´ll have to do some experimenting as well.
Here´s what my installment looks like:

VP1 LFO rate mod