Vertical climbers are extremely low impact as far as exercise machines go. There’s no sudden, jarring movements, which are the ones that you have to look out for if you have have weak knees or hips.
The hand and foot grips of the machine are always in contact with you too, which means they’re able to do a pretty darn good job of evening out the stress that’s placed on your joints.
Also, remember that with most vertical climbers you can change the level of resistance that the machine puts out. If you think too much force is going to be a problem for your knees, then simply start on the easiest setting and work your way up. That’s all there is to it.