In my opinion I think it is no ok to eat vegan hot dogs or hamburgers on days of abstinence or fast days,
You're blurring things with the use of the term "OK". Is it sinful? No. Is it ideal? Perhaps not. We need to realize that the primary mortification of fasting is in the WILL, in obedience to the Church. If a person avoids a real Whopper and gets an Impossible Whopper instead, in order to obey the Church, the person is still making a mortification of His will, and it is in the will (rather than in the tastebuds) that all sin resides. Mortification of the WILL is the mortification of all mortifications. So, for instance, eating $20/pound salmon might not be ideal either, but no one has every accused someone of sinning by doing so. As with all things related to the spiritual life, we must always be careful to distinguish between sin (in the will) and an imperfection (lower than the will).