Translation:What is in the bottle?
How would I determine if the speaker is referring to a bottle or bottles in this sentence.
I would assume it's one bottle in this sentence. If there are multiple bottles, the sentence could be clarified by saying "这些/那些 瓶子” ("these/those bottles")
Not necessarily. In Chinese, we can talk about multiple items without using demonstrative articles.
花瓶 - vase
If 花 (flower) is included before 瓶, then you emphasize "flower bottle", which implies "vase". Otherwise, that word doesn't work in the sentence.
Just a joke: I know at least a type of flower that people would put it "in" a bottle - 桂花.
It is a kind of grain sized tiny yellow flowers which are usually simmered with sugar into a floral flavored syrup and sold in bottles.