Remember that all your fence does not need to be raised, for this reason, count the number of posts that you need to raise. It happens because in most suburban areas the height of the backyard fence posts is higher than the front ones.
Then install your fences only after gaining the desired length, do not install those at first, as it may create some problems at later times.
You can install rails for installation help of lattice, fence panels, frames, etc.
First, you need to measure the extended portion of the vinyl fence with U channels. The one thing you need to remember is that you should take some extra lengths for calculation.
The tools you are using to install the channel, make sure that that tool does not harm the channel. The types of tools vary according to the materials of U channels.
Then frame the fence sheet with channels after that place the fence between the rails of the vinyl posts.
After placing those posts then screwed those with channels and provide stable support.
Then use exterior grade screws with stainless steel.
You need to give your attention and care to cutting and measuring the operation of U channels because they will carry your entire structure load. If there is some problem in cutting & measurement then it will create some huge problems.