Alhtough not as warm as in the summer (and with a bit higer chance of getting wet), October is a good time to visit since you're rewarded with less people in the mountain and with a wonderful view of the surrounding forests in their fall colors.
Like most of Limburg's hills, the Vaalserberg is a famous cycling target. The area between Maastricht and Vaals might not be very appealing to a true mountaineer, but the explosive climbs are quite challenging for the road cyclist! The bike is probably the best way to enjoy the hills around Vaals, just make sure you have enough gears on it..
I did it in march, after snowfall. Snow makes this area a little more special. And if you take your own route in the surrounding forrest, it can be a nice tour. So take the right moment and enjoys the highest "peak" of Holland!