The novel states "The Don had told him [Luca] to keep trying but merely as a sideline, not to press it."

I wouldn't call it slippin'. Vito used his best agent to find out what was going on. Luca took his time, he didn't press, he waited until he was approached.

1) This leads me to the conclusion that the notion of the Don's slippin refers to something else: The recruiting of Paulie as a body guard and driver.

2) The other question is: Was Luca slippin'? He should have been aware that the bar staff had gone (contrary to what Bruno had told him that night).

Sollozzo asks that question about Vito's lack of protection. Vito left himself relatively unprotected even after acknowledging that he was worried about Sollozzo.

The Don was slippin' when he sent Luca to the Tattaglias with the utter ridiculous story that he was "not too happy with our family." Every Mob guy in NY knew Luca was Vito's man to the end. And, even if he was "not too happy," there's no way Vito would have let Luca ply his unique trade for another family--no way. The ridiculousness of that story must have tipped Tatt and Sol that Vito was worried--and that they could trap Luca.

The "not too happy"-phrase is an abbreviation in the film. In the novel, we read:

That's not too dumb. Let's have a look: Luca had done missions which were far more dangerous. He was smart. But the situation in the bar should have made him alert.

That whole thing with Luca being sent to the Tattaglias always seemed pretty dumb to me as well. But I suppose it was a necessary plot contrivance to set in motion Don Vito's assassination attempt and Sonny's subsequent assassination.

On the other hand, Don Vito may have sent Luca to his death just to see how serious the threat from the Taggaglias really was. Although sending such a valuable asset as Luca when he could have sent someone much lower on the totem pole still seems like a bonehead play to me.

I don't think Luca was the issue.

"The don was slippin'. ten years ago could I have gotten to him?"

I think that line is the key. I think his point is that after making a major refusal to partner up and to not see the bigger picture (Barzini's involvement) Vito leaves himself open and vulnerable like some old man buying fruit at a corner stand with no one but his incompetent son for protection.

Ten years ago, Vito would never have been so complacent after such a meeting. Vito even demonstrated there was something bigger by sending in Luca. So why did he resign to a complacent attitude about his own safety?

Sollozzo was right. The don was slippin'.

As Clemenza told Michael, it had been ten years since the last war. The novel is more explicit about it.