The 27-year-old forward -- an integral part of Liverpool's Champions League success last season -- has started the campaign in flying form scoring a double in last Sunday's win over Arsenal.
Vogel, though, told the German SPOX website he wondered whether Salah had the mental strength to succeed when he asked him to come for a week long trial in 2012 -- on the back of scoring a double for the Egyptian Under-23 side against Basel in a friendly.
"It was always clear to me that he is an outstanding player, but whether he had the mentality? I didn't know that exactly," said Vogel.
"Momo is a gentle killer.
"I would be lying if I had prophesied such a massive global career for him."
Vogel then asked Basel's scouts in Egypt to confirm his potential by taking video footage of him.
"I wondered whether the videos had been speeded up," he said.
"He had everything -– the speed, the finish, the left foot.
"Then he trained on the first day (in Switzerland); everyone watched the session and we wondered if he might have a twin brother."
Vogel, who picks out Arsenal's Swiss midfielder Granit Xhaka as the most impressive of the Premier League-based stars he coached at Basel with Salah and his Liverool team-mate Xherdan Shaqiri the other two, said things progressively got better.
"The second day was a bit better, but not good.
"And then came the third day. It was then that he destroyed everything, he was really unstoppable.
"It was absolutely extraordinary. He was so agile, so explosive. If he had the ball on his left foot, it was a goal.
"After that performance everyone knew why we wanted to sign him."
Vogel, who was sacked on October 2012 only months after clinching the domestic double, said Salah's difficulties were understandable.
"Momo was very sure of himself, but he was immersed in a new world," said Vogel.
"He had to acclimatise, in the most literal sense of the word.
"He came to us from the heat of North Africa.
"It is always difficult when you come into an environment where you don't speak the language."
Salah, though was keen as mustard to learn German something which Vogel did his best to dissuade him from doing.
"He couldn't speak English at all," said the 43-year-old German, who is presently coaching third tier German side KFC Uerdingen.
"He wanted to learn German but I said to him: 'Slow down, champion. German is bloody difficult. Learn English, that'll be enough for you.'
"Momo had a really great agent there who could speak English, so we spoke through him."