If it's a Kdvd you can even use selectable subtitles.
That way they will always be shown correctly.
You just need to "hack" an hexadecimal value with ifoedit so that they keep the original colors.
The other downside is these selectable subs are not anti-aliased like the vobsub/textsub ones.
You choose the way you wanna go.
If you like selectable subs, simply use
VSRip that belongs to VobSub package.
Rip the subs you will want in your (K)DVD and process the video and audio tracks as usual but without any vobsub() parameters in your avisynth script.
Then use
Muxman 0.15P free to multiplex the video+audio+subs.
Cheers