1. Depends on whether they are offering to repair it or total it. You likely need to document all the things damaged and work with a body shop on the estimate or do one yourself. Don't short change yourself on this amount, add everything you can including labor costs. If you have records of items purchased and installed, that will help you.

2. Find comparable VX'es for sale to be able to argue replacement value from something comparable with similar mods.

3. Consider hiring a public adjuster, a professional who works for you to get you the most from insurance. I don't have personal experience with this but have heard about it and it makes clear to the insurance that you mean business.

4. Don't show your cards early, i.e. don't tell them your bottom line value or say anything to them that limits you in the future in case you have to fight them later. Better to ask for more than you'd be willing to settle in the end.

5. A lot of above may not make a difference. Last year, my Trans Am was totaled at 100% other guys fault. After they had given me the initial estimate, i submitted receipts for around $2500 for getting a sports transmission installed. They valued that modification at a laughable $100. Their initial offer wasn't bad but just shows that aftermarket installs don't always get you money from insurance. Not much you can do in such cases unless you are willing to hire a lawyer and go through extra hassle.

6. Finally, and you probably don't need to bring this up right away, but if you decide not to use an insurance provided rental car, then the insurance should pay you what is called "Loss of use" per day, around $20-$40 per day. They usually do it from the day of the loss to until 5-7 days after they give you an estimate. The rules are obviously different in different states but make sure to ask for that too. They don't always offer it themselves but are required to pay this, at least in CA.

Anyways, take all the above with a grain of salt and do your own research. I am not an expert in any way, just sharing a few things I learned last year from personal experience. You'd be best served talking to an expert, perhaps a lawyer who deals with such cases.