If you have two professionals that are manufacturer certified in this product say that it is installed improperly then your next course of action is to ask the agent to deliver a notice of defects to the seller (hopefully you are still within the time frame for your inspection contingency) along with the written reports from the inspector and the professionals stating that this is an improper installation. The seller then has 10 days to respond to this notice with a remedy to the situation or to produce reports from experts that say the install is to manufacturer specifications. If the seller does not offer to remedy the problem then you are out of the deal. If the seller elects to fix the problem and bring the siding install to manufacturer specs, or can prove that it is to manufacturer specs, then you are moving forward with your purchase unless you choose to consult an attorney and even then there is no guarantee that you will get out of the deal unscathed. Giving the seller the right to cure keeps them in the drivers seat, hopefully your agent explained that to you.
If your home inspection contingency has passed, then the situation is very different. By letting that deadline pass without delivering your notice of defects you have essentially said that you are OK with the home as is. Again, this is something a buyers agent would have explained to you.