I would simply use Dark Ages rules. In Dark Ages a shield possibly:
- Lowers difficulty of active parry (defensive action) of the shielder, the lighter the shield, the lower the difficulty to parry. For example, a small round shield (a buckler) lowers this difficulty by 2.
- Gives passive defence against melee and/or missile attacks against the shielder by raising difficulty of the attacks. This is essentially the same as being behind a cover. For example, a kite shield increases difficulty of attacks with both melee and missiles by 1.
For slow projectiles, such as thrown stones, or if you have Celerity, you could probably even try to actively parry a projectile, with Quickened Senses combo discipline (Auspex 1, Celerity 3) you could probably even parry a bullet.
Ballistic shields are relatively heavy, so I would make actively parrying with them (if you don't have another weapon) even harder (+1/+2 to Difficulty) unless a shielder has a sum of Strength and Potence of at least 4.
I would agree with Space Ostrich in that shields can be treated as cover against missiles, as listed on page 278 of V20 Corebook. Dark Ages basically offers the same thing (increases difficulty to hit you).
Shields, even ballistic, differ in size. Some only cover your torso, some can be treated as "only head exposed" (+3 difficulty to hit for the opponent) if not as +4, as some shields don't even expose your head. Keep in mind that 10 is always a success result anyway. However, because ballistic shields are especially designed as cover and normally have designated observation slits and firing holes, I wouldn't add any penalty to the shielder (just like using a shield in Dark Ages). If the shield covers you fully and you are not shooting at the moment, hitting you from the front would be totally impossible.
As one of your hands is occupied, you can't use a weapon that would require using both hands, such as a sniper rifle, only a pistol or a small SMG would be OK. Or a katana. :3
Ballistic shields are a very resilient device, I didn't happen to find a test video which would show some weapon's ability to penetrate it, even AP 12-gauge bullets didn't succeed. I would assume that some very, very heavy guns like stationary high-caliber machine guns would penetrate them, but this is nothing more than an assumption.
Riot shields, on the other hand, are light-weight and don't always offer protection from firearms (though, some protect their user from handguns and ordinary shotgun slugs). If a shield is hit, I would simply add several dice to target's soak pool depending on shield's resilience, but make parrying difficulty relatively low, they are as easy to use for melee combat as medieval shields if not better. The same story with soak dice would go if trying to penetrate a ballistic shield.
A riot shield is something like pavise shield of modern days, so I would apply the same rules for both. Sadly, I didn't manage to find the stats of pavise shields.