Answer: would have come.
Explanation: A verb phrase is the part of a sentence that contains a main verb and all of its “helpers,” or auxiliary verbs. In the given sentence we have two clauses, an independent clause "Leslie would probably have come with us" and a subordinate clause "if she didn't work all summer," the main verb is "come" from the independent (main) clause, so the verb phrase is "would have come" because it includes the main verb and its auxiliaries ("would have").