“Be+形容词”这类祈使句可以用very修饰吗?

——完全可以。你的句子意思：

Be very careful with the vase because it is fragile. 弄这个花瓶要特别小心，因为它很容易碎。

更多例句：

Be very clear about what jobs should be completed, and by when. 对哪些工作应该要完成和应该什么时候完成必须非常明确。

Be very careful with this stuff, it can be dangerous if it isn’t handled properly. 这东西一定要小心,稍不注意的话就会有危险。

Be very careful how you deal with her because she’s a force to be reckoned with. 跟她打交道得多加小心，她可是个厉害人物。