Sorry guys, another long one here but weapons and naval vessels are two things I know a fair bit about . . . Please read on.
I agree that the VCD is somewhat defenseless after it fires it's main gun, apart from it's fighter screen and it's rather clever armour. however, you aren't going to use the main gun in every situation. (Though you may not want the enemy thinking that.) Every time we have seen the main gun fired has, as I recall, either been in testing (A Call to Arms) or as a last ditch if-we-don't-do-this-we're-definitely-done-for style maneuvre.
Compare it if you will to the closest modern day equivalent that I can think of. The aircraft carrier. Unless you have a handy airbase nearby (which the US often do, bunch of rabid colonials that they are

) it is amongst the best solutions in long distance force projection. I admit my knowledge of the proper big carriers is somewhat limited as here in the UK our carriers are somewhat dinky (but who needs runway when you've had VS/VTOL for decades) so most of what I say comes from carriers smaller than the Nimitz class, et al. But I reckon it holds true for most carriers. Most of a carrier's force projection is due to the prodigious number of fighters they carry, as the VCD carries a large number of fighter craft. Unlike the VCD, modern day carriers don't have a "big main gun" equivalent, nor do they have a vast number of offensive and defensive weapons other than their fighter screen (I know they have some, but not nearly enough to hold their own in a fight) but they are still considered a valuable asset in the art of warfare, even though they must be supported and often defended by other, smaller vessels.
I think the VCD is a great ship, obviously you don't want to go firing your main weapon off willy-nilly (as is true so often in life) but when the excrement hits the air circulation device, you'd rather have the option of firing it as not have the option at all. Due to it's array of defensive and offensive weaponry, the VCD is more capable of solo ops on a wider spectrum of missions than our modern day aircraft carriers. But like them, can be made more effective when operating at the centre of, or as part of a fleet of similar and/or smaller vessels.
If you view the VCD solely as it's main gun then yes it is a bit rubbish. Sorry to cross genre but even the Death Star could run away once it had fired.
If you take into account all the additional capability the VCD has besides the main gun, and the sheer psychological effect of having an unfriendly one show up in your system or near your battle fleet knowing what the main gun can do if they decide to use it.
Imagine if you will a new class of contemporary-ish naval vessel, an order of magnitude larger than the current biggest carriers. Give it a massive fighter screen (JSF, FA/18, F-15, F-16, basically any aircraft or helicopters you feel like, even a C-130 if you want) defensive weaponry e.g Sea Wolf/PAAMS, Goalkeeper. Some artillery,, Exocet, Storm Shadow cruise missiles (which kick Tomahawk in the backside but you can have them instead if you want). All well and good, very conventional so far. Not much use too close into the coast, but then neither is a VCD in a dense asteroid field.
Now give this new super carrier an EMP blast device, against which it, and all it's friendly ships/aircraft are protected, but the enemy are not. Not a good "main gun analogy but sufficiently close to illustrate my point". This EMP will disable all electronics (other than aforementioned protected systems) within a suitably large range, thus rendering your enemy's hardware useless. Downside is, if you use it, the power surge required will drain all systems of power for an hour. Now tell me honestly that every Military Chief of Staff wouldn't want one
(with matching velour trim :lol: )
Lastbesthope
"Fear will keep the local systems in line. Fear of this battle station!"
Moff Tarkin, on the Death Star, SW IV : A New Hope