Look like a fairly standard horizontal drop-out to me. I can't see a separate hanger, looks like the mech is atached directly to the drop-out. As Joe suggested the screw is to adjust the wheel alignment, so that it sits straight when you remove and re-install it, several of the frames I had in the 80s had horizontal drop-outs with aligment screws.
With regards to indexing, the shifter and the cassette sprocket spacing need to match up, but the derailleur itself doesn't really matter, and the Suntour VX was a good mech, I had one years ago and did thousands of trouble free miles with it.
http://www.disraeligears.co.uk/Site/SunTour_Vx_S_derailleur_%282500%29.html. One exception is SRAM, some of their mechs require a different cable pull and don't work with Shimano shifters.
If you look at the link that Nigel posted:
http://sheldonbrown.com/cribsheet-spacing.shtml, if the spaces on your 7 speed cassette are 5mm, you should be able to use a Shimano 7 speed shifter with it and your derailleur. I've heard some bad things about cheap grip shifters, but there are plenty of thumb shifters available on Ebay:
http://www.ebay.co.uk/sch/i.html?_nkw=thumb+shifters&_sacat=0&_odkw=thumb+shifters&_sop=2&_osacat=0&_dmd=1 - there are even some Suntour shifters.
Are your gears indexed? If not you could pretty much use any friction thumb shifter, early 90s Shimano and Suntour shifters had a little switch on them, so that they could be either indexed or friction only.