I think the vineyard was Buffy's fault. She did a terrible job preparing and got the result one normally gets when the unprepared go to war.
First, where was the knowledge of her enemy? Buffy did nothing to find out about Caleb or what the Bringers were up to or what Sunnydale's underworld was up to or even what the local humans were doing.
Caleb's entire power is a new thing. They had no way of discovering it before he wanted them to.
Second, she didn't do a good job of training an army. She trained a bunch of individuals to fight in a pack and did not give the potentials the individual fighting skills to survive.
This is a no win situation for Buffy I will not blame Buffy for
She had never been in charge of training anyone. She'd never even thrown a punch before she was empowered as a Slayer. The Watchers had never had to train potentials for fighting in a group, so even Giles wouldn't have a method for what you ask. She could have called in Riley and the Initiative(or whatever it's replacement is called) and they wouldn't have had a method because Slayers don't use modern weaponry and weapons training which plays a big part in military training. Plus, I'm not sure US military hand to hand stuff is considered top quality
Third, she didn't listen to her war council. The vineyard was a pretty obvious trap.
let's look at her War Council
Willow: traumatized, deathly scared of having to use her full potential
Giles: inconsistent, willing to make choices she will disagree with and unilaterally implement them,willing to go behind her back, frequently absent from the house so may not know the full extent of the situation in the home
Faith: previously a traitor, has no leadership experience to Buffy's knowledge, took the girls our partying when everyone keeps talking about how important keeping them safe
Robin: known his for five months, spent most of that time lying to her and going behind her back.
Xander: the most supportive of her, but the least power, the least willingness to try to get more power. Takes big risks fighting, and encouraging the potentials to fight like him might not be protecting them. The second time around is traumatized
Anya: super old, might have a lot of experience but was never a fighter. Most of her battle experience comes in the four years Buffy has known her.
Given what a terrible commander Buffy has been for the last 10 episodes, I was surprised anyone with experience went along.
Buffy refusing to give up command is just assining. But, Buffy has shown such pig-headedness all season, so.......
prior to season 7 the Scoobies had experience in ONE battle where they led a group of human strength people into battle. So, no one had experience
Everyone had told her she HAD to lead them. You can say, well now they're saying she doesn't have to, but that means they were wrong to put her in charge before, if they were wrong before, why couldn't they be wrong now? It turns into a circular argument about why she should trust their advice then? Also, while people like to point out how much she relied on them, generally she was at the center of it. Either it was her ideas being implemented, or it was her body being put on the line
Plus, the way this group was acting, if she was there and she implemented someone else's plan, she'd still get the blame if it failed. She was a distraction, she didn't put her all in it, or even why did she let them implement such a bad plan?