Auckland-born Ekuasi has been a standout player for the Force this Super Rugby Pacific season with his physicality and ability to break through tackles the highlight.
The 24-year-old, who has Tongan heritage, was viewed as a key long-term player by Cron, but Ekuasi will join US Montauban on a two-year deal at the end of the Super Rugby Pacific season.
It comes just two months after French Top 14 side Clermont signed Force lock Darcy Swain, who will leave at the end of the current Super Rugby campaign.
Cron said the Force simply couldn't match the financial deal offered to Ekuasi, who has previously played for the Blues and Rebels.
"He's a massive part of our squad. We're upset about it," Cron said.
"And it's one of the challenges ... about having the dollars to keep guys like Vai in the system.
"My biggest concern is we develop players like that and we're going to constantly lose them to Japan or France, or franchises that have more money."
A devastated Cron labelled Ekuasi a great rugby player.
"I'm gutted he's going, but I understand it from a personal point of view," Cron said.
"He's got a partner and a child, and we unfortunately can't compete with the financial side of what he was being offered."
Ekuasi has been named at No.8 for Saturday's crunch clash with the Queensland Reds at Suncorp Stadium.
The Force (1-5) sit second last on the table and must beat the Reds to keep their fading finals hopes alive.
Former NRL star Zac Lomax has been named on the bench for his Force debut, while Carlo Tizzano (knee) and captain Jeremy Williams (concussion) have been ruled out.
20°C