Defender Vestergaard rifled home in second-half stoppage time as Saints punished Chelsea’s lax defending, with the recalled Kepa Arrizabalaga enduring another day to forget.
‘Turbo Timo’ thought he had won the day by shifting his Chelsea career into top gear, vindicating his £53million transfer fee to open his Premier League account in style.
Kai Havertz then converted Werner’s smart tee-up to put Chelsea back in front, but Chelsea could not hold on and defender Vestergaard forced in the late equaliser.
The out-of-form Kepa’s latest clanger proved costly, with the Spain stopper colliding with the post when Adams scored his goal – and that after finding himself stranded from Kurt Zouma’s shy back-pass.
Chelsea would have toasted an almost immediate lead at the top of a turbulent afternoon, were it not for Alex McCarthy’s smart goalkeeping.
Ben Chilwell was worked into acres of space and drilled in low towards the corner after a steadying touch.
McCarthy was equal to the low effort however, and Saints escaped unscathed.
Kepa conjured a good save to deny Adams after a smart pass from Walcott, but Saints’ attacking raids were few and far between.
Werner quickly had the ball in the net, heading in from a Chilwell cross, only for the offside flag to deny the Germany striker.
The pacy forward was not to be denied moments later however, conjuring a sumptuous turn off Chilwell’s smart pass to leave the struggling Jan Bednarek for dead.
Werner then raced into the box, cut across the face of goal and still found time to add a fine finish.
For a maiden Premier League goal, this was something special – and entirely in keeping with Chelsea’s expectations of their big-money summer signing.
Werner doubled his and Chelsea’s tally just before the half-hour, racing onto Jorginho’s lofted pass and again catching Bednarek cold.
The former RB Leipzig man chested down, chipped over the onrushing McCarthy and then nodded into the empty net.
Chelsea were rightly in cruise control at 2-0, only for Havertz to be hustled off the ball deep in his own half.
Saints latched onto the chance, and the ever-clinical Ings delivered a fine finish.
Chelsea turned around 2-1 ahead when they should already have been clear then, and a snapshot from Ings after the break could easily have seen Saints level.
The ball flew the wrong side of the upright though, handing the Blues a hefty reprieve.
Saints did eventually level, with Adams blasting high into the net. Chelsea only had themselves to blame however.
Zouma’s under-hit back-pass left Kepa stranded, and the Spain keeper clanged against the near post amid a game of six-yard box pinball.
Adams proved the sharpest amid the mayhem, latching onto the loose ball before drilling home in style.
Kepa’s shot at redemption only came about so soon due to Edouard Mendy’s thigh injury, and the 26-year-old will doubtless rue another missed opportunity to reassert his Stamford Bridge status.
The aggrieved Blues at least vented their clear frustrations in a quick-fire move to regain the lead.
This time Werner turned provider, laying the Blues’ third on a platter for Havertz, who dinked home neatly.
Saints refused to let Chelsea coast to the win, with Adams firing wide after smart approach play from Theo Walcott, who impressed on his first Saints appearance in almost 15 years.
Kepa produced a good save to deny Ings, but Chelsea could not find the fortitude to hold on.
After a half-cleared free-kick, up popped Vestergaard with a low drive, that eluded everyone – to floor Chelsea and send Saints into wild celebrations.