This radiator vent used for and how is it used

If you're not going to have ventilation (which I'd reconsider .. stain and polyurethane are pretty stinky materials to use, a lil air might help you avoid a nasty headache and potential health issues from inadvertently huffing) an electric heater would be best. Kerosene heaters are going to produce additional fumes which is going to add to an already smelly situation. I'd go for an electric model and keep it as far away from the piece of furniture as you can. Here is a good page on electric heaters. I'd seriously consider having some sort of ventilation though just because of the odor and potential fumes if the stain and poly get warmer, they're likely to give off more fumes than if they're cold.

This is basically going to be landlord work unless you plan playing heating technician and checking the basement pipes. Here's the gist:

• Air vents on the radiator must flow air out so it can be replaced by steam to heat. They sometimes have adjustment valves on them so you can balance the system.
• Air vents that are producing a lot of noise are either
  • too small of a vent
  • clogged up / broken (this is you because you're leaking steam)
• Improperly designed systems can also push a lot of air out of the radiators. The steam delivery pipes should also have air vents on them before the radiators.

When working properly, those air valves should not let steam through. They should close when the steam reaches them, and open when there isn't steam. If there are showing visible steam, they need replacement.

I found this to be a good article on caring for steam vents that will provide more detail.