To answer the slipcover question, no, none of them come with slipcovers, probably due to the fact that the cases are “Double Alpha Keepcase”, and are therefore much wider than your standard DVD case. The two two-movie collections are housed in standard DVD cases (Single-Width Keepcase), but still do not come with slipcovers.
I haven’t seen all of them, but I do own “20,000 Leagues Under The Sea”, “The Parent Trap”, “Old Yeller”, and “The Love Bug”. In terms of bonus material, “20,000…” is clearly the best. It is packed with bonus material, including an hour and a half making-of documentary. I wouldn’t hesitate to say that it is one of the best DVD’s the Disney Company has ever released, which is fitting, considering that the movie itself is easily one of the greatest Disney titles ever.
While not as packed as “20,000…”, the DVDs of “Old Yeller” and “The Love Bug" are still pretty good in the bonus features department. Each one comes with a 35-40 minute “making of” doc, and various other featurettes. “The Parent Trap” DVD is something of a disappointment. While it contains a lot of bonus material, it doesn’t have a really good comprehensive “making of” documentary, which is something I always look forward to on DVDs/Blu-Rays. The making of is only nineteen minutes long, which pales in comparison to the ones on the other titles I’ve already mentioned. This is particularly disappointing for me since “The Parent Trap” is my favorite Disney movie. Still, like the other Vault Disney titles, there is a lot of bonus material included with the movie.
That’s about all I can say from personal experience. Here are some more detailed reviews covering each of the films you’ve mentioned:
http://www.dvdizzy.com/20000leagues.html
http://www.dvdizzy.com/oldyeller.html
http://www.dvdizzy.com/oldyeller-2moviecollection.html
http://www.dvdizzy.com/pollyanna.html
http://www.dvdizzy.com/swissfamilyrobinson.html
http://www.dvdizzy.com/parent.html
http://www.dvdizzy.com/parenttrap-2moviecollection.html
http://www.dvdizzy.com/lovebug.html
In addition, these sites also contain reviews of the DVDs you’ve mentioned:
http://www.dvdmg.com/index.shtml
http://www.dvdtalk.com/