The Faenza-based squad introduced the tweaks on the first day at Barcelona and, despite the poor weather conditions, the Frenchman said he could immediately feel the difference.
Vergne completed 59 laps on his way to the third quickest time of the day.
"We have new upgrades here [and] we are really happy," Vergne said.
"There were many things we wanted to test and we could follow the programme quite well despite the bad conditions.
"In the wet I think we were looking competitive. In the dry at the end of the day it was looking more or less OK.
"It was only four laps so it was quite tricky to have a good feeling, but just in those laps it gave me good feedback."
The Toro Rosso driver refused to go into details about the upgrades, but said there were "little things to come" over the rest of the test.
Vergne admitted, however, that it was still too early to judge where Toro Rosso is in the pecking order.
"It's impossible to say [where we are]," he said. "You never know what fuel the other guys are on.
"The only thing I know is that we have a better car. I prefer to wait until Melbourne qualifying to say anything more."
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