Sol wanted Tom to get the message first hand because he thought that Sonny was now the head of the family, with Vito "dead." His threat that it would be "bad luck for you" if Sonny did not make the deal was a collective "you," meaning the corleone family.

Don't forget at this point in time it was Barzini behind Sol, not Tatt, as everyone believed. Barzini's ambition was to become the top Capo in New York, something that would have been easy with Vito oout of the way, thus in the truest sense Tatt and Sol and even Sol's drug deal were stalking horses for Barz.

Had Vito died, Sonny would have lashed out and ended up dead just as he did even though vito lived. Bottom line, Santino was a bad don, and being businessmen, Clemenza and Tessio would have ceded to Barz leaving Sonny dead, and Tom hanging out there with no muscle. Tom would have either disappeared to Vegas or somewhere or been killed hiself.