Its a shame Sony Vegas crashes . it is one of the better programs. Ver 7, and now 8 are good at editing Mpeg and don't lack for the ability to add title text music ect.
Converting to DV AVI is questionable as you are going from a compressed format to a less compressed one, only to go back to a compressed one. However for extensive editing it may be an option provided you have the HDD space, and you do your project in bite sized chunks. (ie
One dvd at a time ), then join them together at the end.
4 DVDs worth of video seems to be a fairly ambitious project .Pruning unwanted footage from each one should make life easier for you
Im a bit

: There is no reason why you should end up with 30Gb per DVD. Even with DV AVI from tape the most you use up per hr of tape is 13Gb
Some software are native "mpeg editors" in that they are able to avoid unnecessary re rendering of mpeg clips which don't need it.
As you know, compared to DV AVI, mpeg files have worse "generational" PQ loss with re rendering
Some of the mpeg editors use "smart rendering" to get round this.
Ulead Video Studio is said to be able to "smart render". It is (perhaps justifiably) very popular but some on these forums have been underwhelmed by its output. and GUI
Womble, Video Redo and DVD Santa are other titles which are said to be good at DVD/ mpeg editing
I have in the past used Womble for simple cut, join type edits of video from DVDs but not for titles, transitions music ect.
I think I have an older version but it isnt in the league of the Full blown mainstream NLEs for features
Ulead movie Factory 6 ( and 6 plus) is mainly a DVD authoring title with some editing abilities.. It is worth exploring if its trial version does not also crash on you

Other than that, all the currently available software titles for editing can edit footage from DVD largely because its recorded format ( mpeg2) is the same as that from HDD camcorders which are much more commonplace now
HTH