Vendrick, 71, disappeared on Feb. 16, 2008. He had left behind his laptop and his medication in a hotel room, and his rental vehicle was found abandoned nearby.
Now, his business partner Gary Shawkey, 47, awaits trial for his murder.
"Mostly, I'd like to know how it happened and why he did it," said the victim's brother Fred Vendrick.
The informant in the case, who only wanted to be identified as Biggins, said he wanted to provide answers for the victim's family. Biggins said he met Shawkey in jail in 2009.
Starting in 2004, authorities allege that Shawkey, the self-proclaimed motivational speaker from Virginia, took more than $1 million from the Arizona retiree.
"They were always in the context of, 'Hey, you invest some money with me, we're going to be able to make a lot of money,' and that's how he got the victim to give him money," said prosecutor Ebrahim Baytieh.
But investigators suspect Vendrick had had enough, and he wanted his money back.
"This startled Shawkey. As a matter of fact, Shawkey told me it's the first time he considered killing Vendrick was approximately six months before the actual incident," Biggins said.
Surveillance video showed Vendrick in an Orange County store on Feb. 15, 2008, the day before he disappeared. Shawkey had earlier bought a 23-foot sailboat called the Odyssey, along with a new engine, a depth finder and a new anchor.
Biggins alleges Shawkey admitted he lured Vendrick on the boat by lying about a trip to San Clemente Island to close a secret government deal.
"He was finally excited about finally getting his money back," said Fred Vendrick.
Biggins showed a drawing allegedly done by Shawkey that showed a 100-foot rope. One end was tied to the boat, and there was a clasp halfway to attach to Vendrick's belt. The other end held an anchor.
"He trusted Shawkey explicitly," Biggins said.
Biggins said Shawkey told him he convinced the inexperienced sailor to wear a belt, calling it a safety device. On the way to Catalina when the water was at least 1,000 feet deep, Shawkey pretended the engine died, and he asked Vendrick for help.
"He throws the anchor in, and as the anchor is taking the rope down, he clasped Bob, picked him up and body slammed him in the water," Biggins said.
Shortly after, he pulled Vendrick's body up to confirm that he was dead, and then dropped him back into the water, according to Biggins.
The prosecutor won't go into detail, but he suspects Vendrick was thrown overboard. Despite numerous searches, Vendrick's body has not been found, and a boat anchor is also missing.
In the past, Shawkey told investigators Vendrick started feeling sick, so they returned to shore. Shawkey claimed he dropped him off on the dock.
Vendrick was soon reported missing by his wife. For months, Shawkey provided authorities with several explanations where Vendrick could be. One story was that Vendrick was in Mexico to start a new life.
But prosecutors believe Vendrick is dead.
In 2009, Shawkey was arrested on an unrelated matter. The murder charge for Vendrick's death came weeks later.
"He's a murderer. He killed him for financial gain," Baytieh said.
So far, Biggins is not expected to testify. The prosecutor said he's confident in their case without him.
Shawkey has pleaded not guilty. His attorney declined an interview with Eyewitness News.