First of all, it looks to me like you do have some slow, static units in your list. Your opponent, if you have not already intercepted his entire force with your mech units, is likely to try and reach your static troops with his assault/close range units. Fortunately, support weapons can be a very decent counter against this threat.
In a mech list, I'm not sure vibro cannons would be a decent choice because your grav tanks are fairly large, and therefor might get in the way. It's possible that your opponent will find ways to actually hide behind your tanks to avoid vibro cannon line of fire.
The shadow weaver is a very cheap option point-wise. It provides you with long-ranged support, but that might not be used to its full potential when the enemy is closing in on your static firepower troops, which will probably be close to your support weapon platforms I'm assuming. The other thing about shadow weavers is their lack in psychological effect. They are not really that powerful, so the enemy might just ignore them alltogether..
I think D-cannons would suit your list perfectly. These tank-killing/heavy-armor-ripping guns can be the ultimate psychological defense against oncoming assaulters. They pack an amazing punch in just 150 pts worth (100 pts can be decent as well). If you keep them close to your static troops, I guarantee your opponent will think twice about entering the d-cannon zone.
anyway, hope that helps.