Long answer :
Virtualdubs subtitle filter can not be used to mux subtitle streams into an existing AVI. It will only allow you to kind of 'add' subtitles to the picture if you are reencoding a movie. The subs will become a part of the picture then, and can never be removed ( unless you are using other filters and reencode again, degrading quality further and leaving strange coloured fields where the subtitles used to be ).
Actually, the incapability of AVI to hold subtitle streams in one file together with video and audio, was one of the main reasons ( amongst others, like missing support for VBR audio ) to start development of the MCF container about 2 years ago. While Tobias Waldvogel's OGM implementation based on Ogg does allow even this already today, the MCF project was suffering from a project fork, resulting in 3 projects with similar goals now :
OGM
matroska
MCF
IMHO its just a question of time until we will finally be able to stop using AVI, once one of these containers gets wide spread use, is documented properly and fully editable/usable in Virtualdub
!!Read more about these new A/V containers aiming to replace AVI in the 'new A/V formats' section here, in the 'Important links' thread ...
Christian
BTW : The developers of the VirtualdubMod project are investing a lot of their free time into adapting new container formats in their excelent modification of Virtualdub, and it will even allow to add subtitles to OGM files today ....
