Darren,
The tension on corners can be relived by making small cuts in the seam allowance. The bulk does not always give enough and this will cause the material to pucker.

You can also apply some heat to the area to help relax the vinyl. Steam is the safest way, but you can use a heat gun if you are careful and do not overheat the surface of the vinyl causing it to get shiny.

Many thanks Fred. I’ll give steam a go on this job

I have the same question for armrests like this. Is it my pattern or my sewing? TIA!

Try steaming from the underside. I find it's best on curved areas like this to cut the vinyl on the bias which would allow it to stretch in all directions. You may want to cut it slightly tighter so the material can be pulled to fit.

It might be the direction of the stretch in the vinyl. Vinyl must be patterned correctly so that when it is installed it will lay on the surface padding and conform.

You might try using a heat gun to warm the material to help it relax into position. Work the stretch up until the vinyl cools, reheat and then forward or down until it cools.

Do not over heat the vinyl or you will get shiny spots. A rule of thumb is- if it is too hot to the touch, you have overheated the material.

Thank you!!