Well for one I don't think there's anything in Traveller to suggest that maneuver drives are capable of anything approaching significantly fractional-c velocities. In fact it's strongly suggested by the background that they top out at a velocity of 6G x 3.5 days. Because if you can't get there in a week or so (half acceleration and half deceleration) then you jump.
Personally I also feel the whole vector preservation through jump later addition is bogus on several levels and that, again supported by the way the game plays, jumping will zero your velocity relative to the destination. So no building up a big vector and jumping in with it to surprise your target.
Besides, in the setting history this scenario has happened... never iirc. There must be a reason or reasons for that. Physics, society, or something else is enough to mean it doesn't happen. Since society is a weak control I'm more of a mind that there are solid physics behind it being practically impossible.
Well, yes, - and no. Here's how I see it:
Interplanetary space is not empty. It has a lot of junk, most of it little sand-grain-sized and smaller stuff left over from comets flashing past, asteroids knocking into each other, the gods alone know what else. Lets use Earth as an example: Earth sweeps up something between 35,000 and 70,000 tons of this stuff annually during its orbit - but Earth is a bit bigger than the average scout/courier.
Figure - with many, many rough approximations to simplify the math - the Earth at ~1.3 x10^14 square meters cross-sectional area, sweeping a circle with a radius of ~150 million kilometers and encountering, say, 50 billion grams of "grains of sand" over the course of a year. So, there are 50 billion grams in a volume of some 6x10^25 square meters - ballpark the puppy at roughly a gram of matter for every 10^15 square meters.
My courier has a cross-sectional area of 90 square meters. I should on average encounter about a gram of mass every 10 billion kilometers. In other words, I'm only expecting to encounter about 0.03 grams of mass in my inbound flight. Now, 0.03 grams - 30 milligrams - can actually make for a lot of little grains and flecks if you figure a grain of sand, for example, might be anything from 0.6 milligrams to 0.3 micrograms or less.
At 11-1200 kps, 16-17 hours into my flight, 0.03 grams mass is 40-million joules, grains of matter becoming tiny specks of hot plasma needling through my hull and through my ship, possibly to be stopped in the hull on the other side. Not a healthy condition for my machinery OR me when it happens - and my plan is to triple my speed. By comparison, the M1 Abrams' main gun delivers a KEAP round with roughly 25.6 million joules of energy to penetrate 540mm of steel (penetration rating >45) - that is of course a much bigger mass at a much slower speed.
The real question becomes: can these few tiny little superheated, superfast (relative to me) specks do enough damage to stop my machinery before it completes its programmed mission?
How the heck should I know? I'm just a gamer.