I can't help too much with modifying patterns because I tend to just wing it, but a couple things...
If you don't want to start with a sloper, look for patterns that have similar seam lines to what you want to make. It's not too hard to move seams, but if you don't have to then that saves you time to focus on more complex mods. Trace the pattern you're using so you can keep the original to trace again in case something goes wrong (and it probably will, don't get discouraged, happens to everyone).
If you break up the ideas you have into smaller changes then it becomes less overwhelming. A pattern book can help you figure out the basic shapes and techniques to use, but you'll need trial and error to really know how different things work with different fabrics. (for example here's a book section on sleeves. This is my favorite resource though the page is now archived. http://web.archive.org/web/20070204015930/http://vintagesewing.info/1940s/42-mpd/mpd-04.html
It's info for vintage styles, but the same techniques apply to anything you want to make).
I've sewn a good bit with vinyl. The biggest challenge is that it probably won't feed through your machine without a fight. I like to layer it between two pieces of tissue paper to get it going. Unless the vinyl is super heavy (think upholstery), I don't think there's a need for leather or heavyweight needles. Medium weight needles have always worked fine for me and leave smaller holes in the vinyl. When you're sewing it,the bigger/closer together the holes the more you weaken the vinyl so using a bigger needle might not be best. You also usually only get one chance at a seam because any stitching you remove will leave holes (pins will too, so either don't pin - use paperclips, tape, etc., or pin inside seam allowances.) Each "unusual" material you use will have it's quirks (heck, each "normal" fabric does too) but you can often google "sewing (type of fabric)" to find tips.
Modifying patterns and using weird materials can get really frustrating when things don't work, but it's also really rewarding when they do work, so don't give up!