I own Mongoose Traveller Core Rulebook, in which there's everything essential you need to play any Major race-characther...
Vargr Drifter,
Well, that's patently untrue because as you say...
... it doesn't say a word about that Vargr couldn't kick.
... meaning everything essential to play a Vargr isn't in
MgT.
Given Mongoose's track record this should not be a surprise.
The Vargr have
always been unable to kick in the same manner humans do because of the
structure of their legs and not solely because Vargr are digitigrade and humans are plantigrade. Unlike humans, Vargr knees do not bend forward. While a digitigrade gait requires a different limb than the plantigrade gait does, some digitigrade animals like elephants do have knees that bend forward.
What
MgT says on the matter is of no concern because Mongoose has screwed up
again.
Traveller canon, as derived from all real
Traveller versions, has said the Vargr cannot kick like humans from the beginning.
At the Hivers paragraph it tells same way with thick black text: "Hivers can see infra-red spectrum."
That's something Mongoose
almost got right. The Hivers have three IR sensitive sensors equally space around their head limb and alternating with auditory sensors between adjacent eye stalks. As repeatedly described in
real Traveller, these IR sensor patches provide
limited vision into the IR spectrum and the sense separate from and not integrated into the Hivers' regular visual wavelength sense.
So, Hivers don't "see" the infra-red sepctrum. They have a limited IR sense instead.
I bought Gurps Alien Races 1 though...
The Vargr descriptions in
GT:AR1 are far more accurate than those in
MgT.
There it says that they couldn't kick for their on-toes walking-style...
Again, it's not just their digitigrade gait, but the type of leg structure a digitigrade gait usually requires.
... but I follow basicly my corebook and take the extra backround stuff from Gurps book.
What you do in your
TU need only be justified to you and you alone. You can change canon in any way you see fit.
If there would be people in my group who would own just the corebook, someone would want play Vargr and he or she would be like: Huh?

o: That's not fair! You are abusing your GM-power!

"
Then all you need to be is "like":
"Huh?
o: Mongoose routinely screws up! They abuse their license power by ignoring thirty years of canonical information primarily through sloth and indolence. I'm the GM and I've chosen to correct Mongoose's shoddily researched materials. You need to realize that the Vargr have walked this way longer than you've most likely been alive."
If you take the time to explain to your players why something is so in your setting, they cannot complain. And in this case, your "change" from Mongoose's information is actually a correction.
By the way, which other races walk by that on-toes way?
Droyne, Aslan, and several Minor Races, but it's not a digitigrade gait alone that prevents kicking like humans do. It's also the leg structure the digitigrade gait usually requires; the "knee" bends backwards, not forwards.
Regards,
Bill