Somerset completed a convincing 95-run victory over Essex in their Clydesdale Bank 40 semi-final at Taunton on Saturday to keep alive their hopes of a double-winning season.
The Sabres, who are also chasing their first LV= County Championship First Division title, will meet Warwickshire in next weekend's Lord's showpiece after the Midlanders beat Yorkshire by four wickets at Scarborough.
Somerset skipper Marcus Tresco-thick, who took the side to last month's Friends Provident t20 final, losing to Hampshire, was again at the heart of another impressive batting display from the Sabres as they racked up 312 for six batting first.
The former England opener plundered 79 from 62 at the top of the order to lay the foundations of a target swelled by some enterprising late hitting. Nick Compton weighed in with 55 but the fireworks came from Jos Buttler (36 from 18 balls) and Arul Suppiah (42 not out from 20 balls) at the death.
Essex lost Mark Pettini from the first ball of their chase and with key men Alastair Cook and Ryan ten Doeschate both falling with the score on 33 they never truly recovered. The last five wickets fell for just nine runs.
Warwickshire ended Yorkshire's own bid for a trophy double as they chased down a revised target after rain of 260 off 37 overs with seven balls to spare.
Darren Maddy proved his side's match-winner after they lost three quick wickets. His side still needed 23 to win off the last three overs, but the issue was effectively settled by Maddy who hit Steve Patterson for consecutive sixes over mid-wicket and long leg. Rikki Clarke was bowled two balls later, but two fours for Maddy off Anthony McGrath in the next over sealed the win.
Earlier, Yorkshire opener Jacques Rudolph scored 106 from 95 balls, adding 99 in a partnership with Gerard Brophy (64) for the fourth wicket.