Subrip positioning isn't supported
VLC doesn't support the positioning "tag". FFmpeg support it It may be a good idea to implement it since
Here is an SRT file which contain an positioning "tag".
1
00:00:00,000 --> 00:00:10,000 X1:100 X2:100 Y1:100 Y2:100
Example
If you wanna see how FFmpeg implemented it, see this function.