Your dvd is problaly set to progressive mode. and vcr video are not progressive. If you don't want to keep switching cable, you could just watch dvd in composite mode also (yellow, white ,red) but you will sacrify some video quality because of that though.
No answer for problem 2 because I am not too clear as to what's happening.
Before I start, let me say that I don't know much about HDTV and all that stuff.
Hi, I bought a Magnavox 15inch HDTV that also is a LCD monitor. I also bought a Samsung DVD-V550. Ok, I wanted the best quailty picture I could get, so I bought these HDTV cables.Supposedly all three were for video, red, blue, and green, I think. I had some` regular sound cables, the red and white. Will that work ok? My DVD player has a digital audio ouput, but my TV doesn't, so I used the regular calbes. Now, these regular sound cables had the yellow, regular video, cable, but I didn't connect it. Is that ok, or do I need to buy another cable that has just the AUDIO, red and white? OK, so I can play DVDs always, but not always with sound. It is really hard to switch between DVD and VHS. But when I try to play a VHS, it doesn't come. It just plays the DVD's screen saver, nothing. I tried connected the AV cables, and the VHS works fine, but why won't it work in HDTV mode with those other cables? If I missed anything, please ask since I'm new and may have forgotten some key thing. Thats the first problem, the second is this: I want it connected to my pc, which I can see fine, but the LCD also has an audio jack for PC sounds. What kind of cable do I need for that? It looks like a regular audio jack that all PC's use. Thanks!
-Ibrahim
*CNET Help forums need an edit button!

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic