The Steelbacks finished second from bottom in their nine-team group, picking up just six points along the way.
They finished with a four-run defeat to Leicestershire Foxes at the County Ground on Thursday evening.
And Vasconcelos said: “It has been a campaign of what ifs.
"Firstly the Yorkshire game (which was rained off), we felt we could have got on and got over the line and then we’ve had really close games at Notts and today.
"If those go our way we qualify.
“Our powerplay bowling has been excellent throughout but our batting has not been good enough.
"Our top order hasn’t had enough guys taking it to the end and winning games for us.”
The Steelbacks threatened to chase down Leicestershire's 271 all out but in the end, the hosts could not get over the line.
"It was tough throughout the day, it was pretty even," Vasconcelos said.
"They got off to a flier and then we pulled them back, which was the same in our innings.
"Unfortunately we couldn’t get over the line."
Leicestershire, like the Steelbacks, were unable to make it past the group stage of the One Day Cup.
And Foxes head coach Paul Nixon said: “We thought 10 points would be sufficient.
"It got close and at one point when Glamorgan were going nicely then we would have gone through but it wasn’t meant to be.
“There were a lot of good performances today. Rishi (Patel) batted beautifully, (Louis) Kimber is hitting the ball nicely again and almost had to rein himself in because we were two wickets too many down and (Rehan) Ahmed batted maturely for his 40.
“I’m delighted we’ve won three on the bounce, and four in the competition, but sadly it wasn’t enough.
"We wanted to get through but by a whisker we haven’t.”