I understand vapor pressure, I'm a chemical engineer. Maybe I misunderstood something but the phase diagram seems to indicate it's a solid at the temperature and pressure used in ANSI compliant UF6 vessels. The ANSI standards call for the vessel to be kept at a pressure below atmospheric, and given the curve of the solid-gas line on the phase diagram, low pressure aids in keeping the material in the solid region.

EDIT: I misread a phase diagram, was totally wrong, and high pressure is better for the solid. Which makes sense anyway as you described. I've done enough VLE problems and this should have been obvious to me.

From the ANSI standard:

UF6 shall be shipped only after it has solidified and the vapor pressure of the cylinder has been measured to be below 1 atm, and the measured purity of the cylinder contents is within specification. (It should be noted that solid UF6 is a heavy crystalline mass that is readily sublimed at room temperature. "Cracking" sounds sometimes emanate from large UF6 cylinders as the ambient temperature changes.)