If you genuinely could not fit in Standard Class (not just "it's a bit crowded", physically not being able to comfortably fit), then your argument is fine. If however, you CAN go into Standard, even thoguh you have to stand in a semi-crowded space, then you shouldn't be in the First vestibule.
After all, where does the guard draw the line? If you stand there and think it's perfectly fine, then ten others see you doing it and come to join you (then blocking legitimate passage by FC ticket holders, for example to the toilets, or if FC is also full and there are FC standees) where do you draw the line? Would you say you are a special case?