Great: on vacation : Dogs are always in vacation.
Correct:  Dogs are always on vacation.
Explanation: We use the expression 'on vacation' to show that someone or something is on a holiday or is away from school or work.

Great: When summer vacation comes, I take vacation by going swimming pool.
Correct: When summer vacation comes, I take a vacation by going to a swimming pool.
Explanation: We have to put an article 'a' before the word vacation. Also, we have to add some words to make our sentences clear. It should be, 'going to a swimming pool' instead of going to swimming pool.

Great: When I saw melten ice-cream, I can't believe my eyes.
Correct: When I saw molten ice-cream, I can't believe my eyes.
Explanation: Be careful with your spelling.^^ It should be molten, not melten.

These are correct (Good job!):

take a picture : My little brother is taking a picture.
get in : I get in the taxi.
look for : I am looking for my lost money.
do one's best : When you take a test, you'd better do your best.