I'm not sure I could say what "type" they are, because that would just be hypothetical fan fic.
But to be more specific, the main phasers on the SNS Galaxy have a strength of 1400.  The Vivace class will have the same phasers as the Supremus with a maximum strength of 2500.  So the phasers themselves have quite a bit more punch to them.  That being said, the max charge of the phaser banks on the SNS Galaxy is 1.5, whereas those on Supremus and Vivace have a max charge of 1.  All three ships share the same discharge rate.  So really, if the SNS Galaxy had the same charge characteristics of my ships, it's strength would be equivilant to 2100.  
So to sum it up, the phasers on Vivace are definitly more powerful than a Galaxy class, but I also envision that the Vivace and its sister ships are commissioned in the late 2380's, a full 30 years after Galaxy Class starships hit the scene.