Try sampling different vinyls.  Some upholstery vinyls stretch better (2-way) than others.  A heatgun helps.  Or, use 4-way stretch vinyl , which may be hard to find at stores.  Ask trimmers & suppliers for a source.  Try Dap Weldwood top & trim adhesive.  

Stretching is always the best way. Heat guns help a lot here. If you can't stretchit anymore then you can try this. Sewing Sewing can also make it easier. You just have to make sure it looks right. First take 2 pieces of fabric. Cut both pieces along the edge of whatever it is you want to cover, then sew them together. This works well on rounded corners and wavy areas. Be careful not to cover the entire car like this becase you start to get that Frankenstien look if you do. Because this is so easy some people tend to get carried away.