I am under the impression that the only time it is morally acceptable to act is when such behavior is deemed to be morally acceptable.
This statement is critical. Right off the bat it isn't clear what moral standards you're referring to. Are you simply stating that according to any given moral theory, it is morally acceptable to X when X is a morally acceptable action
according to said moral theory?
That would be tautologous and uncontroversial. However, your following statements lead me to believe you're sneaking in this proposition under the guise of a tautology: According to any given moral theory, the only time it is morally acceptable to X when such behaviour is deemed morally acceptable
by a particular set of people
Note that while the first statement is entirely uncontroversial because of its tautologous nature, the second requires justification to hold. Your further statements lead me to believe it is this second proposition you are attempting to use here such as in this case:
Blowing up an abortion clinic in protest of abortion is immoral because it violates the code of morality that a society has in place.
This assumes that the right thing to do is in fact to abide by the code of morality society has in place, regardless of any objective moral truths that may exist. While this can be justified by some moral theories, it still requires justification, and it is far from clear that
all moral theories would produce this conclusion.
Consider this scenario:
There is only one objectively true moral theory, and you are its only adherent, and you know it to be true. According to this moral theory, you are morally required to blow up an abortion clinic.
By definition, the right thing to do in this instance would in fact be to blow up the abortion clinic.
Now, we might feel that it is wrong, and socially acceptable practice may be to refrain from blowing up the abortion clinic; but the intellectually honest response to this situation would not be to insist that blowing up the abortion clinic is the wrong thing to do. We could instead say that while the right thing to do is to blow up the clinic, in order to remain a functional member of society and to live a happier life, one should not blow up the clinic.
But what if, in your gut, you felt that something ought to be done and yet this was not accepted by society? Should you become a moral vigilante?
I would say, no, you are not morally permitted to be a moral vigilante because your opinion of what is moral may differ from the opinions of everyone else in the society. If you don't like the morality of the society you live in, then either gtfo or peacefully demonstrate to try to change the accepted morality.
Otherwise, to be a moral vigilante means to assume that your opinion has more worth than the opinions of others before it has been established by democratic vote or social meme.
This section on the other hand is positing that moral feelings and beliefs are simply opinions. In other words, it is denying that individuals are (at least at present) incapable of knowing an objectively true moral theory. However, this does not address the argument I have made above. I concede that for the vast majority of people, acting on what feels to be moral or what they believe to be moral would likely not be the right thing to do. This would depend on a host of other factors, but the most important one would be that they likely do not have knowledge of the objectively true moral theory.
In summary, these are the important points I want to convey:
1) The introductory lines are misleading, implying that what is said follows from the first interpretation, rather than the second
2) If one adopts the more robust first interpretation, what follows is that in the extreme case where the objectively true moral content is diametrically opposed to what the vast majority of people believe to be the moral content, the morally right action remains dictated by the objectively true moral content and not by what society believes to be the moral content.