Buffy seems communicationally challenged here. When the objections started, if she had said, "But we are not going to do it the same way", people would have shut up and listened.
Buffy's mistakes here were in 1) not working out the details of her plan with Faith and the Scoobies before she ever said anything to the Potentials. That way, she could have been assured of their support 2) picking the moment that Xander came home from the hospital to announce her intention to return to the vineyard, and, most importantly, 3) not waiting until Spike returned. Whatever your feelings for Spike might be, it must be remembered that he was one of the two most powerful fighters in Buffy's team, and his strength might have been crucial to whatever plan she had devised (but we'll never know, because she was never allowed to reveal any of its details)
They had raided a supermarket and a hospital for supplies. How about a gun store? Did anyone ever try to decapitate a vampire or a bringer with a shotgun?
Unlike grocery stores, gun shops don't just leave their merchandise out on display when the shop is closed for extended periods of time. Firearms are generally locked securely in safes or vaults. Besides which, we have never had any indication that anyone on Buffy's team, with the exception of Xander and hie pseudo military experience, has any knowledge of how to safely and effectively handle guns.
How about a fuel truck? Vampires can be burned.
This, they should definitely have considered, especially since Buffy's original reason for coming to Sunnydale was that she burned down her old school gym in order to kill a nest of vampires.
Had the army base bugged out yet? They may have left, expecting to return and they may have left weapons.
Military arsenals are even more secure than most bank vaults. Unless Giles or Wood has safe cracking experience, that would be a lost cause (the way that Xander was able to steal the rocket launcher in Season 2 was a total joke, by the way. It could NEVER have happened like that)
I think the worst part is their plan once Buffy left.
They didn't think of a different way of fighting. They just marched in to get the weapons they were told by the Bringer were there. And no one thought it was reckless or idiotic or dangerous.
The irony is, Faith's plan in raiding the supposed arsenal was almost identical to Buffy's plan to go to the vineyard: blindly go in without reconnoitering the area and just assume that you can handle whatever might pop up. No surprise that the end result was pretty much the same as well.
What we see here is a case of everyone condemning Buffy for her initial mistake, but the only one who seemed to learn anything from that mistake, at least until making the same mistake again, was Buffy. This is why I really wish we had known what her plan for the second assault on the vineyard actually would have been.