I know in some mixes you add white to get it colours to look older but with red it seems to me it would just turn it pink.
Depends on the red. Even depends a bit on the white.
Working directly over grey primer/basecoat can help with achieving duller colours but since you've already undercoated in red here are some suggestions:
mix up a duller base red than you would normally and apply a thin, even layer of this over the waistcoat;
now mix two or three values of a buff colour (white + dark brown), then dot the wear areas and highlight zones with a lighter mix, the shadows with a darker mix;
using a clean, soft brush stipple away until the dots have been blended in, wiping the stippling brush clean periodically to prevent it getting too much paint on it;
apply some fresh dabs and repeat the process if you think it's necessary and then leave it to dry.
That should give you a good basic washed-out/worn looking version of the dark red you started from.
I'm being deliberately vague on paint colours here as I don't know which ones you have available or the shade of red you're looking for, but the basic method is easily adaptable.
Einion
