Overnight leader Brandon McNulty was dropped on the final climb but dug deep in a damage limitation effort that salvaged a slender four-second lead over fellow American Matteo Jorgenson, with pre-race favourite the Belgian Remco Evenepoel in third at 36sec.
The expected penultimate stage showdown was reduced to a skirmish due to the ultra-short 104km route, but Sunday's finale in the Nice back-country mountains could create a bigger shake up.
Saturday's finish up the Madone d'Utelle 15km climb at 5.7 percent saw the field gradually whittled down with Bora-Hansgrohe rider Vlasov going out alone with 5km remaining.
Resurgent Ineos rider Egan Bernal was bereft of his sherpa Carlos Rodriguez and lost time, while Evenepoel had Yves Lampaert until late in the climb and was first across the line from the yellow jersey group.
Known as the 'Race to the Sun', the 2024 Paris-Nice was shortened on Saturday due to snow and ice in the mountains and was contested in cold, rainy conditions.
Sunday's stage is still a tough one with potential for a shake up with five climbs on the 109km route.
