Deductions & credits

No.....again...it depends on the assessed tax on your tax return.  (Tax owed BEFORE taking any withholding, or other prepayments, into consideration)

The way tax returns work is that, at tax time, you put in  all your income, and all your deductions and exemptions, and then a total tax is assessed against what remains.  Your monthly paycheck withholding is ignored until that calculation is done.  Normally, your Monthly withholding is then used as a credit against that assessed tax....but when the  vehicle credit is available, that credit will be used first to attempt to reduce the final tax result to zero...but no lower than zero.

Examples:
1)  Maybe at tax time, your assessed tax is already zero  (withholding hasn't been considered yet)  because you have so many deductions, and so little income.  The Vehicle credit can't be used...but since you also had withholding, you will get all your withholding back...but no more.
or
2)  The assessed tax is (perhaps ) $5000 for your situation.  Then $5,000 of the vehicle credit is used up to take that assessed tax to zero, and you lose the remaining 2,500.  Again, your get all your withholding back, because the credit was used and took care of all your assessed tax.
or
3) The assessed tax is (perhaps) 8500 for your situation.  All of the  $7500 vehicle credit is used first to pay off that, with $1000 remaining to be taken out of your withholding....and you get a refund of whatever withholding that wasn't used up.

Now, what SJ was saying was that if you can project your approximate tax assessment for 2017....and if you find that you are in situations #1 or #2...then it is an opportunity to convert some of a Traditional IRA to a ROTH IRA, such that the credit is used against what you would normally have to pay taxes for the conversion....and still get all your withholding back......but to do that, you need to have a really good idea of your total situation for 2017  (if buying in 2017) or 2018 if you are waiting until next year