There is no way of even guessing that and not only would IVing such a mix be incredibly bad on your veins but as you don't know how much fentanyl is in it you are at a very very high risk for a fatal OD. Also if you need a acid to be able to shoot up a drug it would be much safer to get some vitamin c. Actually the guy at the needle exchange here said alot of people use the vitamin c for breaking down the fent patches for injection. They hand it out at the needle exchange here so people don't have to resort to using vinegar. I have only heard of people shooting crack with vitamin c or vinegar.
But yeah no matter what way you take the fent from the patch there is no way of knowing the dose. Even with the gel patches you can't judge how much fentanyl you are taking as it's not distributed equally throughout the gel in the patch. Not to mention as fentanyl is such a potent opioid you can't exactly try and guess the dose without risking ODing and possibly dying because of it. I used to get the 25ug patches for abit and eventually the 50ug patches and my advice would be to wear the patch (if you have the tolerance to even be using it that is) and if you absolutely need to abuse it put some heat on it to get it to release faster. That is the safest way to abuse fentanyl patches though it is by no means safe.
Personally i find fent to be virtually devoid of euphoria. It is great for pain and has little in the way of side effects compared to morphine but i get no real euphoria from it at all. I would much rather dilaudid, morphine or even oxycodone if i wanted to get high.