In as far as I know VobSub only displays
graphics-based subtitles (
bitmaps). [I'm not 100% sure as I don't use VobSub myself; so as you say it also displays .smi files it is quite possible that you're right and I'm wrong, so don't shoot me then
]
On the other hand BSPlayer supports natively
text-based subtitles, like SubRip .srt-files and MicroDVD .sub-files. (You can see that these subtitles are text-based by opening them in e.g. Notepad).
sami or .smi is also text based but not currently supported by BSPlayer.
A .smi file can however, very easy be converted to e.g. a .srt file.
Have a look at
this post and perhaps also
this one.