.srt is not supported. No subtitle format is. Hard subbing is your only option.
PSVita uses the same video types PSP 3000 models supported - AVC H264 with AAC or MP3 audio.
The PS Vita screen resolution is 960*544, though you may need to adjust your exact resolution to be 16:9 or 4:3 respectively for the video you are re-encoding.
Any old video converter will do the trick, just make sure it hard codes the subtitles. I suggest freemake video converter. I convert all my psp anime on that and it's brilliant. - for PSP anime...
You will want to adjust the default PSP settings on that program though, try around 700-1500kb/s depending on how much "action" there is in your anime. If there is a lot, try closer to 1700 or even 1800kb/s.
Your only option for audio is AAC, but you can use MP3 audio if you use a different encoder (or at least you could with PSP 2000/3000 models)
Be sure to use "Stereo" output, not "original" or "mono"
44,100khz for your audio sampling rate is sufficient. Use a bitrate of 128kb/s, anything higher is not necessary as the speakers or your headphones will not replicate the quality anyway, so it's really just wasted bits.
As long as you have some basic encoding knowledge the rest is easy. That program has to be the simplest gui I have used, hence it is no good for regular encoding, but audio and small screen encodes are brilliant (anything below 10')
Another program I use is XMedia-Recode. I have not tested if it works for PSP yet, but assuming I used the right codec settings it should also work. Hard subs are possible with this gui, however it's a pain to get them to work. Usually they end up soft subbed.
Minicoder can also be used, however, it does NOT encode to a format PSP likes, so it can only be used to hard sub the stream, after that you will need to further encode it using a different program. If you do use this method, set minicoder to an output bitrate of approximately double what you really want. On the second encode, you can reduce this. If you do not follow this method, you may see some major blocking even on the tiny 5' PSVita screen.
Edit: If you do use freemake to "burn" or "hard sub" the subtitles in, please note that unless they are in .srt format, you may end up with the command quotes being included
Click here to see an example, a simple mux of the subs, open in aegisub or subedit and then save as .srt will fix that. Just be sure to mux it back in to a copy of your video before you encode it, and remember to remove the original subs, or you may end up accidentally encoding them. Actually, freemake doesn't even need the subs to be muxed into the video, you can select your source right from the editing window
The program also features a [limited] frame cropping function. You can use this to remove the op and ed to save
1 space
2 time
when you watch your anime (unless you are like me and love to watch the op/ed every time)
The above guide should see you having about a 150mb output video. - assumption, I have only ever used these settings on a PSP, so I am not 100% sure of the output size having never encoded to that size. I usually end up with 130mb for my normal psp encodes
Edit 2: There is also a program out by red kawa, the official pspvideo 9 software, that should work for vita, though it has set settings, so you may not be able to get the resolution you want. It has been a year or 2 since I last used it, so I am not sure if it hard codes. Just thought I'd mention it as a reference if all else fails.
Hope I helped
-K-
EDIT**** DOES NOT WORK WITH 10BIT ANIME. You will need to use XMedia-Recode for that. That program works a charm for encoding back to 8-bit
