Before I even begin commenting on the models, I have to talk about the picture quality.
I cant make a true C&C unless you take better pics. Using a flash washes out all detail and changes the color of the models.
Look at the difference flash makes on these two models.
The next problem with the picture is the background. Use a solid background or at the very least, move everything out of the way.
I would also suggest taking the pic with a dark/black background instead of the white. When using the camera in the auto setting, like most of us do, the camera see all the white (paper) background as 'light', so it closes the iris/aperture.
The result is that your model comes out darker that it actually is because the aperture is now closed to compensate for all the 'light'
Assault Squad:
First thing that I notice is the helmet. They have no eyes/lenses. They look unfinished without the eyes. You can get away with this on heads, but not on helmets.
Also, you need to add some metallics to the helmet, specifically the mouth pieces/respirators on the face plate.
The torsos have too much metal in them, you need to break up a little. It seems 4 of the Marines still need their eagle painted gold.
I would also paint the jump pack straps black, just to break up all the metal.
You can also either drill out the barrels or simply paint a black dot on them.
Did you wash the metal? It's hard to tell with the flash.
Vindicator:
You can still see the brush strokes, or rather you can see the black primer under the brush strokes.
You're having the same color issues that you have with the troops. It's very monochromatic. Give it some more color.
Paint the head lights, the sensor/targeting array (on top), paint the barrel red or some other color.
Maybe some weathering on the vehicle. Just a very light stippling or drybrushing of brown along the bottom to give it a dirt/dust effect.
Wash. Brown or black.