The second list looks a little better.
I'm not sure about Mephiston being the
HQ... Specially if you want him to take las cannon fire, he's more points than the Vindicator. Might be better off taking a cheaper
HQ and just a 3rd Vindicator, even if it can't have the shield.
I would split one of the Assault Squads into 2 squads with Rhino's or RazorBacks. Use the Rhino/Razorback as
LOS blocking/cover for the jump pack unit and bikes. Move up 12 with everything, shoot your bikes/jump pack unit, then use flatout move to get in front (In which case Rhino's might be better since they're cheaper).
Edit on comment on Frag Cannon, was thinking something else...