* "Very" is an intensifier: It amplifies the meaning of the word it modifies. So, the opposite depends on what word it's modifying.
Here are some possible antonyms, depending on the context:
* Slightly: "The cake was very sweet." --> "The cake was slightly sweet."
* Not at all: "The room was very dark." --> "The room was not at all dark."
* Barely: "He was very tired." --> "He was barely tired."
* Moderately: "She was very happy." --> "She was moderately happy."
* Not very: "The movie was very good." --> "The movie was not very good."
To find the best antonym, think about the opposite of the meaning you want to express after "very."