Everything that's been said above is true. But to address the question as to why an average soldier was not sent instead, well that would have been to much of a risk for Vito to take.

Vito knew that he could trust Luca with not only carrying out the mission that he was sent on, but more so in that Luca would not turn on him or even tell anyone else in the Corleone family what Vito sent him to do.

Had Vito sent an average soldier on that type of a mission, he not only would have ran the risk of that soldier actually turning on him, but he would have also ran the risk of that average soldier running his mouth off to one of his associates which could have and probably would have meant big trouble for the Corleones.

Luca was Vito's most loyal and trusted friend.

Unfortunately Sol and Tags, or should I say Barzini, knew this.

It may have been a risk that Vito had to take. But as said above, he was slippin.