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Porte crossed the line less than 20 seconds behind the Russian who finished third overall at last weekend's La Route d'Occitanie, just 23 seconds in deficit to the Ineos 1-2 of Egan Bernal and Pavel Sivakov.
The Australian performed strongly at Occitanie himself in sixth place overall and is looking good ahead of the Tour de France late this month.
"The guys were awesome today," Porte said. "They believed in me, and every one of them did a super job.
"It's nice to be second on the top of Mont Ventoux. I think it's a good sign for the next month."
On the 20km climb to the finish, team mate Tom Skujins drove the pace to under eight kilometres to go before Kenny Elissonde took over.
When Cofidis' Guillaume Martin leapt from the pack, Elissonde led the chase with Porte and the major contenders following.
Vlasov landed the killer blow, his attack decimating the pack as he passed Martin on his way to victory.
Porte then pulled away solo, dropping Nairo Quintana (Arkéa Samsic), passing Martin but unable to bridge the gap to Vlasov.
"Look, of course, you want to win," Porte said. "But Alexander was very strong today. I think he’s a super talent. I am happy with second, it’s a great result.”
“I am happy with where I am at before the Dauphine, that’s the bigger goal,” continued Porte.
"I felt good on Ventoux, that’s probably one of the hardest climbs we do, and the heat also affected us. I think it’s a good sign where I’m at going forward.
“It was a great race – fast all day – but also strange to race with what happened in the Tour of Poland yesterday. Fabio (Jakobsen) was always in the back of my mind, and thinking about the fight that he has ahead of him.”