Ok, Vallejo make good paints and in the main they'll behave in the same way you are used to with the Citadel paints [I actually use a mix of the two ranges.] Their Game Colour range is also analogous in shades and names to the Citadel range making it an easy switch.
In my experience however, when I first tried out Vallejo 5-6 years ago, their metallics and their greens were pretty poor; this may have changed.
P3 use a liquid pigment as far as I can tell. I have a few of their paints for fine detail work in certain shades [whites and off-whites] because this gives a smooth strong colour. The downside is that this means the paint mixes differently [i'd suggest buying a proper thinning medium to use with them rather than water.] Also their range is based on triads for their own range. So to find equivalent citadel colours you'll have to do a bit of trial and error.
If you stop using citadel you will lose out on the foundation range though and imo this is a BAD thing. [I love their washes but these can be more or less replicated using a matte medium; for example liquitex.]
Honestly, the easiest switch is probably Vallejo, plus they're pretty cheap and easy to get you hands on. I wouldn't abandon Citadel completely though if they do the better paint for a colour/purpose you want.... that'd be cutting off your nose to spite your face.
Hope that helps some?