If your mini dvd's are rewritable RW type and you have finalised them to play off the camera on a DVD player I think you will need to unfinalise them in the camera to use again, if they are write only mini dvd's R type I don't think you can use them again.
I would use Nero Vision Express, part of my Nero 6 cd/dvd burning suite myself as explained here, however you may not have Nero on your pc
If you don't have Nero you could download vob2mpg free
and use it to convert your vob files to mpeg2 files then use whatever dvd video burning software is on your pc to create a dvd from your converted mpeg2 collection, obviously up to the maximum size available on a blank DVD disc.
I'm not sure whether vob2mpg needs the files it's going to convert to be on the hard drive first or whether it will work direct to a DVD or mini DVD, if you need them on the hard drive this site explains the dvd structure
You could open one of your mini discs on the pc, right click the disc then left click open, open the video T_S folder and starting with VTS_01_1.VOB drag them to a folder on your hard drive and later use vob2mpg to convert a file one at a time to mpeg2.
Just drag the video title object set files the VTS_01_1,then2,?,?series.VOB files to the hard drive,( you can check they are the right ones first by clicking on them and they will play in your pc's dvd playing software) they will take a little while to copy to the hard drive, as you are going to create a new DVD structure later with the mpeg2 converts you don't need any other files.
You could probably combine 3 of your mini dvd discs in one DVD later.
There are other free dvd ripping and authoring tools online like DVD Shrink or a trial version of VideoRedo, but with you having about 20 discs, I think I'd do the drag one at at time then convert to mpeg2 method myself.