In my games as GM, I settled on a “soft” approach to dealing with these kinds of weapons, making it more about target selection and “component destruction” or weak-point targeting than about straight damage.
Things with low armor and HTT can be fairly easily dealt with by moderately heavy weapons or even lighter weapons that can beat the armor (e.g., a blaster rifle vs. an armor 1 speeder), but if you’re going against an AT-TE or something of that nature, it takes a more specialized approach.
For one thing, larger vehicles will be of much less “universal durability.” While some may have extremely heavy armor in certain areas, others will remain vulnerable. For the AT-TE, this would be the articulated segment between hulls. In these situations, I simply change the armor and would usually require a “Called Shot” Aim Maneuver. I might, instead of reducing armor, name an “automatic” or “minimum” crit that would be generated if a crit is triggered.
I sometimes abstract this with an appropriate Knowledge (usually Warfare) check, but just as often make it an RP thing.
The larger the vehicle, the more distinct and individually targetable its components are. While it would be quite difficult to target the steering vanes of a speeder bike, you could target the leg of an AT-AT with much less difficulty (though a rocket launcher’s efficacy there is questionable to say the least).
As far as adapting or creating personal anti-vehicle weapons, I once toyed with the idea of a missile launcher that dealt planetary scale damage, but was inaccurate against smaller targets and took more time to aim. I eventually removed it from my sheet because I decided it was no longer needed and was not to my standards of quality. If I were to stat it now, I would do so thusly:
Gunnery; 3 (Planetary); 2; Close; 8 Enc; 2 HP; Limited Ammo 1, Breach 2, Guided 2, Prepare 2, Targeting Silhouette 3.
“When attacking a target of silhouette 1, gains the Inaccurate 2 quality.”
This gives it a degree of anti-air support as well as just anti-vehicle, because air units will only infrequently be measured within personal scale.