"I put in chicken and vegetables."
鶏肉と野菜を入りました should be correct. や is used to denote "things like", as in "i put in things like chicken and vegetables". But the English here states "I put in chicken and vegetables", those are the two ingredients, so とand not や seems more appropriate. At least both should be accepted.
For posterity's sake: The meaning has a bit more nuance in Japanese. You use や to denote that not all things are listed. You use と when the whole list is known. The Finnish 'ja' has no substitute, except comma to list things until the penultimate list item just like in English.