Hi,
I've got like 30 videos that I need to upload on facebook, but they are really big (from 90 megabyte to 300).
So I'm wondering if there is a way, via virtual dub, to compress them all at once.

And if so, which settings you would suggest me.

I don't believe it's possible to incorporate 30 videos into Virtual Dub and shrink them down into one video. I've mostly used Virtual Dub as a compression freeware program with the Xvid codec to compress a single 900mb to 1.5 gig video file to around 30 or 90mb. You have to be careful with the settings since it can really distort videos at higher levels of compression.

Your "best" possible bet is to shrink the videos down and then bring them into a video editing program to combine them into one entity. You could try for Sony Vegas, but I haven't had good success with Virtual Dub compressed videos and Sony Vegas. It seems to throw a hissy fit and wants the videos untouched. Even if you use the Xvid codec in Sony Vegas, you may still end up with a 500mb+ video.

If it were me, I would just use Virtual Dub to shrink them down to 10 to 50mb with the Xvid codec and upload them to youtube, and then make an autoplay list while linking the first video to your FB account.

To begin, thanks for your answer. I wasn't clear. What I wanted to do is avoid to do the same passages for every video. So, basically, I don't want to repeat the steps of opening a video, finding the codec, compress, wait and so rebegin the process for the remaining 29 video.
What I want to do is do this at once. So I wish just to select all the video at once, and compress all the 30 videos without having to do countless times the same steps.
Am I clear, now ?

Batch Processing is what you need then. https://www.youtube.com/watch?v=kCfU148DGSA