I was so excited for this feature, but so long I haven't had success with it. I'll tell you what i've tried:
-vobsub subtitles embedded in mp4 container
-IDX/SUB (DVD subtitle format) along the video (not in container)
-ttxt (timed text subtitles) embedded in mp4 container
-simple srt file along the video (not embedded in container)
-SUP file (Bluray subtitle format) along the video (not in container, not possible)
-XML/PNG along the video (not in container, not possible)
-Separate (Not muxable) XML file (text)
-Separate (Not muxable?) DFXP format
and none of the above works. I was betting on ttxt to be almost the sure option, 'cause is the only way of making official subtitles on the mp4 format, but right now I'm at a loss.
Anyone tried something else? Also, the vita manuals have been updated, but no mention of the supported subtitle formats.
PS: Still trying combinations