We had our first game last night (the "Guadalcanal Finale" scenario) and as far as six experienced gamers could tell reading through the rules as written destroyers are invulnerable at Night and at Flank Speed at any range, broadside or not. At Target Size 6+, Night effectively makes them TS 7+, and Flank Speed makes them 8+. At Long Range it's +9, and at Extreme Range it's +10. (I'll give you the last one: without radar fire control no one is going to hit a DD at full speed at 30,000+ yards.)
It all depends on the conditions you're facing at the time, which is why I don't think the rules have anything to apologize for....
A destroyer is 6+ to hit, but if you have it targeted via the "large silhouette" rule then it drops to 5+.
Put a spotter plane above it, with the large silhouette, and it drops to 4+.
If it moves at flank speed, large silhouette, plane it moves back up to 5+.
If you have it at flank speed, without the silhoutte, but with the plane, it's at 6+.
Flank speed, silhouette, no plane, 6+
Flank speed, small silhouette, no plane, 7+ -- which is fine with me.
It goes on and on. But there are going to be situations where you have a very solid chance of hitting a DD, and times when it will prove almost impossible. And at night, when it's moving at flank speed, a destroyer SHOULD be almost impossible to hit. Let's note that in confined waters, at night, you wouldn't see many real life destroyers tearing up the water at 35 knots, especially when they're mixed up with enemy forces.
If you want house rules to cover a perceived gap in the rules, maybe add in searchlights for night actions? Allow ships to be illuminated by searchlights if they're within 10" of an enemy vessel, and allow for a +1 bonus to hit. But make it a double edge sword (IE, if I announce my cruiser is illuminating an enemy hull 10" away, then I get a +1 bonus to hit it, but then again, enemy ships receive a bonus for attacking me because I just revealed myself). There are examples of ships that carelessly snapped on their searchlights just getting PLASTERED.
OR limit just how fast ships can move around at night to avoid collision.
Or try an open-ended result house rule. If you roll a "6" on a D6, then follow that up with a second toss. If you get a "4," then you landed a hit on a target that would have required a 7+. VAS follows that mechanic with critical hits. Why not try it here?
Final note -- destroyers, to fire their torpedoes, are going to have to have their port or starboard arcs presented to you, which means they will always being suffering from the large silhouette penalty, doesn't it? So look here. A destroyer, moving at flank, but presenting a broadside arc to you in order to launch a spread, is going to be 6+ to hit in daylight. Use searchlights to negate the night time firing penalty, as mentioned above, and you'll always have a chance to hit it, even at night.
Really, I don't see a problem.
