Is this AFTER render and seen on the delivery view? Or while viewing onto an external monitor?
My issues occur when I take too small a font and enlarge it - viz pan/crop or Track Motion. If you know you are going to "reduce" a font start of BIG . . well BIG! It works for me. try out some test of the same font but at different sizes.
Bottom line, for me, is that my White fonts on a black background are gorgeously perfect - Vegas does this very well indeed.