will mercury vaporize in boiling water in a flask?
Mercury evaporates to some extent even at room temperature: Vapor pressure: 0.002 mm Hg at 250C (~300 K). "Although the vapor pressure of elemental mercury is low [at room temperature], an atmosphere that is fully saturated with mercury vapor contains approximately 18 mg/m . The levels attainable in
indoor airs at room temperature can therefore greatly exceed
safe levels and result in poisoning." Mercury metal stored under water in an open container will still release Hg vapor into the air, as it is insoluble in water. Since mercury vapor, and worse yet, mercury compounds that might be formed are so toxic, I would not suggest experimenting with it except with proper guidance and with proper equipment, such as a negative-pressure hood feeding into a scrubber and a containment area for possible spills.

At one time, mercury was used to treat STD's and was used by hat-makers to preserve furs, with serious neurological damage to the users. It accumulates through the food chain to cause some fish to be toxic. If feasible, avoid using mercury in the lab and substitute a somewhat safer chemical.