The liquid water is max 100°C (right?), beyond that it should vaporize (right?)
Yes, but strictly no. Firstly the boiling point of water depends upon purity and pressure.
The purity factor is argued as a reason for putting salt into water - increasing its boiling point and hence cooking faster. In practice this has negligible effect (a brine strong enough for a wet cure would still only be about 102°C boiling point, so the effect on cooking time of a bit of salt is going to be overwhelmed by the pressure factor).
The pressure factor has two practical effects. One is that if you try camp-cooking high in the mountains, it's harder to cook anything or make a decent cuppa, because the boiling point is so low (okay, not that practical unless you climb very big mountains, but some people do). The other is that pressure cookers cook faster, because of the corresponding effect of using high pressure to raise the boiling point.
Secondly, liquids won't necessarily boil when they reach their boiling point. This has a practical cooking safety effect, in that if liquid is heated in a clean smooth container (hence no nucleation sites) in a microwave it's possible to get it to do this. Such super-heated water has enough latent heat to turn the water into vapour, but hasn't done so. Once you do something that gives it a nucleation site (blow on it, knock it, add something to it), it suddenly flashes into steam splashing steam upwards and boiling water outwards, with enough force to smash the container as well as the obvious scald hazard.
In practice though, barring the superheating case (won't happen in a pan) and half-way normal weather and altitude, then yes, 100°C.
Water vapor could be hotter than 100°C (but how much, in normal cooking conditions?)
Not in this case. It can in the cases mentioned above, but not in normal pan boiling.
As you add heat to water starting at, say, 20°C, that heat will cause the temperature of the water to rise. Each large calorie (kCal, the same sort unit used to measure the energy content of food) absorbed will raise a kilogram of water, 1°C.
Once the water reaches 100°C, yet more heat energy is needed to turn it into steam. This takes around 540kCals per kilogram - much more than the amount needed to raise the water by 1°C. Hence the water remains constant at 100°C for a while, then some of it turns into vapour. Now, it only takes .48kCal to raise vapour by 1°C, but that vapour is going to rise, moving it away from the heat-source and helping distribute the heat energy more evenly throughout the water (which after all, will be cooling elsewhere).
For this reason boiling water will be staying pretty much at an even 100°C (not precisely so, but precisely enough for cooking purposes.
Likewise, while ice can be much colder than 0°C, ice mixed with water will stay around 0°C as the heat absorbed goes into melting the ice rather than heating the water.
When boiling water, the vapor originates at the bottom of the pan
Some flashes near the top, but most does, yes.
So technically the foot could be "hit" by this vapor, thus being heated above 100°C
No for the reason given above. It's worth nothing that when the food is hit by vapour, that vapour contains more heat energy than the water at the same temperature, and while it can't raise the temperature any higher than 100°C, it can theoretically do so more quickly. However, liquid water is a better conductor than vapour water, which mitigates this. In all, this has no effect upon the cooking process, but it does explain both why a steam burn can be much worse than a liquid water burn - it's not normally the potential for vapour being above 100°C, so much as the greater heat energy to transfer - and also why one can put ones hand in domestic steam for longer than in heated water - the poorly conductive steam, mixed with air, is not as good at transferring heat, and hence causing injury, as liquid water is.
In all then, boiling water is 100°C, and this isn't a concern for cooking - if the recipe says to boil it, just boil it. The only real concern is to not let it boil dry.