Glycerine & fatty acids @ 180'C and requires to seal at full vacuum.
The valve currently installed has a glass filled seat which is being damaged after 3 months service and will therefore start to pass.
I like your idea of a plug valve.  You can get a higher-grade sleeve than virgin TFE for abrasion resistance, but with a plug valve there is SOOO much sealing area,and it tends to wipe itself clean. A plug valve such as a Durco Sleeveline lets you adjust the plug to restore the shutoff because the plug is tapered.  Since it is on a vacuum service, you might check off on the specs that you want the supplemental o-ring stem seals.  On a plug valve the primary seal to atmosphere is the circumferential band between the top of the plug and the sleeve.  There is a supplementary diaphragm seal that is energized by process pressure (not vacuum) so the supplemental o-rings above THAT would prevent air inleakage. 
The actuation is a consideration: A BIG double-acting rack and pinion, or if you need spring failsafe a scotch yoke actuator would be required.  You might also think about a "Dribble control" package, to partially close the valve near the end of the batch, and to let it snap closed from a trickle flow exactly at the end of the batch.

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