how can i make it only show 25 keys? it goes to 24 but not 25.
You can't. VMPK shows only groups of 12 keys. If you select at the preferences dialog 2 octaves, that means 24 keys. If you select 3 octaves, that would result in 36 keys.
Well, it wasn't possible until version 0.6.0
Now you can choose any number of keys, and also the starting key.