I think Vito knew (as Tom reflects in the book) that his "no" would cause trouble. He probably wanted to meet with Sollozzo face to face to take the measure of the man. He also may have hoped that by doing Sollozzo the courtesy of at least hearing him out that a negative response by Sollozzo or the Tattaglias could be avoided.

"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming

Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.