Peter's vision had nothing to do with food... notice that it was stated it was a "vision". This denoted metaphorical language, not literal. Also, we have the witness of Peter as to what the vision signified. In verse 17 of Acts 10 Peter is pondering what the vision he just say could mean.... it wasn't "obvious" to Peter that it referenced food, because he knew the vision was a symbol of something. In verse 34, Peter states what it is that he now understands the vision to mean... that God is no respecter of persons.
As a second witness to the meaning of the vision, we are given it in chapter 11, verses 1-17..
1 And the apostles and brethren that were in Judaea heard that the Gentiles had also received the word of God.
2 And when Peter was come up to Jerusalem, they that were of the circumcision contended with him,
3 Saying, Thou wentest in to men uncircumcised, and didst eat with them.
4 But Peter rehearsed the matter from the beginning, and expounded it by order unto them, saying,
5 I was in the city of Joppa praying: and in a trance I saw a vision, A certain vessel descend, as it had been a great sheet, let down from heaven by four corners; and it came even to me:
6 Upon the which when I had fastened mine eyes, I considered, and saw fourfooted beasts of the earth, and wild beasts, and creeping things, and fowls of the air.
7 And I heard a voice saying unto me, Arise, Peter; slay and eat.
8 But I said, Not so, Lord: for nothing common or unclean hath at any time entered into my mouth.
9 But the voice answered me again from heaven, What God hath cleansed, that call not thou common.
10 And this was done three times: and all were drawn up again into heaven.
11 And, behold, immediately there were three men already come unto the house where I was, sent from Caesarea unto me.
12 And the Spirit bade me go with them, nothing doubting. Moreover these six brethren accompanied me, and we entered into the man's house:
13 And he shewed us how he had seen an angel in his house, which stood and said unto him, Send men to Joppa, and call for Simon, whose surname is Peter;
14 Who shall tell thee words, whereby thou and all thy house shall be saved.
15 And as I began to speak, the Holy Ghost fell on them, as on us at the beginning.
16 Then remembered I the word of the Lord, how that he said, John indeed baptized with water; but ye shall be baptized with the Holy Ghost.
17 Forasmuch then as God gave them the like gift as he did unto us, who believed on the Lord Jesus Christ; what was I, that I could withstand God?
The issue in chapter 11 was the brethren were unhappy with Peter because he associated with a gentile and when Peter told them of the vision he had, the rejoiced because now they could see that God was not calling the gentiles "unclean" and that they could take the message to them. Nothing in the two chapters dealing with this vision, does it insinuate it is talking about what foods we are able to eat. Of course, that hasn't stopped countless millions of Christians from following the desires of their heart and reading into it what they want.
