Why would anyone shooting at the vitals suffer a penalty to hit? The vitals are in the center of the torso. Someone shooting at the torso would also be shooting at its center. You are going to be shooting for the same place in either case.
To correct for this, you can use the following rules ...
A hit to the torso with an impaling, piercing, or beam attack has a 1 in 6 chance of striking the vitals. If you are shooting at the torso and succeed by enough (see below) you can choose whether or not the vitals are hit, no roll needed.
A hit to the head has a 2 in 6 (1-2 on 1d6) chance of striking the skull hit location, a 2 in 6 (3-4 on 1d6) chance of striking the face hit location, and a 2 in 6 (5-6 on 1d6) chance of striking the neck hit location. If you are shooting at the head and succeed by enough, you can which of these hit locations are struck.
You are assumed to be striking or firing at the center torso unless stated otherwise. A center torso shot has no penalty to hit. The actual hit location struck depends on your margin of success:
0-1 | random | |
2-4 | torso (1 in 6 chance of striking the vitals) | |
5+ | vitals (or no chance of striking the vitals) |
You can alternately choose to attack other hit locations.
Head - You are at -2 to hit. Where you hit depends on your margin of success:
0-2 | random | |
3-4 | head (1-2 skull, 3-4 face, 5-6 neck) | |
5+ | attacker's choice of skull, face, or neck |
Limb - You are at -1 to hit. Where you hit depends on your margin of success:
0-1 | random | ||
2+ | targeted limb (1 in 6 chance of striking the associated extremity). |
Hand or Foot - You are at -2 to hit. Where you hit depends on your margin of success:
0-1 | random | |
2 | associated limb | |
3+ | targeted extremity |
In melee combat, if your opponent tries to parry your blow, a margin of success of 0 means you hit his weapon arm if the parry made the difference to whether you hit.
Personally, I do not like rolling 3d6 for hit location. I find it is faster to use a single die. However, you cannot get enough discrimination on 1d6. Thus, I use the rather un-GURPS like mechanic of rolling 1d10 for random hits, with a suplementary 1d6 for determining special effects of hits to a specific hit location as described above.
1d10 | Location | 1d6 | ||||||
1 | 2 | 3 | 4 | 5 | 6 | |||
1 | Head | Skull | Skull | Face | Face | Neck | Neck | |
2 | Right Arm | Hand | ||||||
3 | Left Arm | Hand | ||||||
4 | Torso | Vitals | ||||||
5 | Torso | Vitals | ||||||
6 | Torso | Vitals | ||||||
7 | Right Leg | |||||||
8 | Left Leg | |||||||
9 | Right Leg | Foot | Foot | |||||
10 | Left Leg | Foot | Foot |
Attack | Die for hit location |
Punch | 1d6 |
Kick | 1d5+5 (roll 1d10, divide by 2, round up, and add 5) |
Reach C melee weapon | 1d6 |
Reach 1 melee weapon | 1d8 |
Reach 2+ melee weapon | 1d10 |
Ranged attack | 1d10 |