The sentence "Die Delfine haben ihn umzingelt, aber nicht vor, ihn zu fressen" makes no sense at all. This is an incorrect sentence. Whereas "Die Delfine haben ihn umzingelt, aber nicht um ihn zu fressen" is correct. However, I had to remove the comma before "um". There cannot be a comma in this position!
The only thing that I can think of is that the first sentence is missing a word. If you change it to "Die Delfine haben ihn umzingelt, hatten aber nicht vor, ihn zu fressen", it all at a sudden makes sense. I added in the word "hatten", the past tense of "haben". The verb "vorhaben" is a separable verb and means "to have the intention to do something". In this case, the sentence means "The dolphins surrounded it/ him/ her, but they had no intention to eat it/ him/ her".
