SergeantBlake

MSVCP100.dll
Download that and put it into your GTAIV root directory.

As in you don't want to replace vehicles like police, police2 and so on?
If so, then it's impossible to add it to the game. If you want to add a police vehicle, you will need to change one of the police models in Open/SparkIV. 
One thing you could do is pick a vehicle that you don't like and replace it with the vehicle that you want.
Please note, if you're going to do that, you will need to rename the vehicle files to that vehicle.
I also cannot ensure that ELS will work.
BACKUP THE ORIGINAL VEHICLE IF YOU'RE GOING TO DO THAT!

For vehicles, you will need a program like SparkIV or OpenIV.
 
SparkIV Tutorial
OpenIV Tutorial
I CANNOT STRESS THIS ENOUGH.
 
BACK. UP. YOUR. GAME. FILES.
to do so:
 
open up OpenIV/SparkIV if your in sparkiv go to the side and scroll all the way down to the bottom, click on CDI images. if your on openiv, you should see models.  export all .img files in there and put them in a SAFE PLACE THAT IS NOT LIKELY TO FAIL ANYTIME SOON. if you dont want to have to replace all vehicles when one stuffs up, back up each individual vehicle
 
If your new, I suggest getting ELS.
Emergency Lighting System (ELS) is what most 3rd-Party vehicles use. It basically animates your emergency light bar and gives you control over your sirens. You can do things like have the lights without sirens and so on.
Kind Regards,
Sergeant Blake.