I cannot say this enough, the Rebels need to do 2 things to win.
1) stay at long range. Look at the comparison of Neb B vs VSD.
Front:
Neb B: 3 Red dice
VSD: 3 Red dice
Sides:
Neb B: 1 Red dice
VSD: 2 Red dice
Rear:
Neb B: 2 Red dice
VSD: 2 Red dice
You have ALMOST the same firepower. When it comes to defense, the Neb has an evade and 2 braces, the VSD has 2 redirects and 1 brace. The Neb can take almost as heavy a shelling as the VSD, provided they are at Range 3.
2) Fighters
In the core box, the Rebels have the advantage when it comes to fighters. Standard Xwings vs Standard Ties, the Xwings take it. Now if you can use your commands to issue Squadron Commands, they will mop the floor with the TIEs. Then they can jump the Star Destroyer. Also do not be afraid to use your ships to shoot down TIEs. I always manage to throw some damage with my Neb B onto the TIEs and then finish them off with squadron commanded Xwings.