The former Springbok captain has been impressed by Monty's inspirational return to international rugby.The goal-kicking fullback proved a key figure in South Africa's Tri-Nations glory this year.
And flanker Vos has no hesitation in revealing the reason he believes Montgomery has been playing such fantastic rugby.
'I think playing in Wales has done Percy the world of good, he's almost like a different man,' Vos told Wales on Sunday.
'Monty's playing some phenomenal rugby and the feedback from South Africa has been awesome. They're over the moon with the way he's been performing.' said the Harlequins captain.
Montgomery joined Newport from Western Stormers in 2002, but the move signalled the possible end to his international career in accordance with South Africa's policy of only selecting players who turned out for their provincial sides.
And Vos, who quit international rugby two years ago, hailed Welsh Rugby as the reason of Montgomery's success.
'It was a huge challenge for him to make a big move to South Wales,' he said.
'Monty was an icon back home and perhaps the fact that he wasn't left alone made him move to a foreign land where he'd be just another person."
Monty's performances have also helped him surpass South African legend Naas Botha's record points tally of 312 as the kicking king reached 358 Test points.