May contain spoilers. Nothing so major but there are various indications.
I could be sarcastic and mean, but screw that. You should discover the meaning of "true warrior" by yourself, that's the beauty of comprehending an art. You have to think and discover your own conclusions, meanings and "truths". Then you can lead long discussions with others about different interpretations of themes presented in the manga/book/anything and if you're lucky enough to find a fine group of people then these debates can be really interesting, amusing and enlightening.
In my case, I think Yukimura presents "true warrior" as an unreachable ideal. A someone who can solve anything by words alone. In 15th chapter (I'm not sure about the number, though.) Thors says "A true warrior needs no blade." but that's only Thors' point of view. Still, we can see that even Thors doesn't see himself as a "true warrior". He's ashamed of himself because he had to rely on a blade (a better word would probably be "a weapon").
On the other hand Thorfinn saw his father as a "true warrior" for a certain time. When Thorfinn was a child, Thors seemed like a ideal human - Wise, kind, hard-working and sometimes strict as a good father should be (Even if we don't like that, we know fathers should sometimes be strict.). The first time Thorfinn sees an "imperfection" in his otherwise "ideal" father is in 4th chapter, when Thorfinn asks "Where would they go if they would want to escape from here?" and Thors doesn't have an answer - "Even my father doesn't know everything." is what Thorfinn probably thinks with sad look in his eyes at that time. He isn't disappointed, he just discovers that there's no perfect human. Later after a "certain incident", Thorfinn is thrown into confusion. Why did his strong father throw away his weapon? - At that point, Thorfinn doesn't understand because he doesn't see things in larger scale, he is too fixated on "warrior" being an epitome of a "someone who uses a brute force".
I just wanted to point out how some characters of Vinland Saga see "a true warrior" so we could get a larger picture. I could mention Askeladd, Bjorn, Thorkell, Canute etc. but in that case I could write for eternity and nobody would bother with reading this post. So I'll just get to the presentation of a "true warrior" out of inner story of this masterpiece.
I believe Vinland Saga is (like Planetes) about searching for a compromise. I mostly see an opinion that Vinland Saga is about "not giving up" but I think this isn't that case. So far we can see many compromises. Askeladd chooses a compromise - He, even though he's a descendant of old British kings, decided to side with Canute and make a king out of him. Thorfinn searches for a compromise between peace and violence, because he realizes that violence is omnipresent. Canute searches for a compromise between kind and cruel king - He wants to be a "good" king, but to lead a kingdom there are sacrifices to be made.
For me personally "a true warrior" is someone, who knows how he wants to live and lives that way. "Someone who searched and found." Doesn't matter what, when or why, he just "found". But I'm not blinded, I'm opened to various opinions and I'm constantly in search. I hope this helped you somehow, I would want to write more but I just don't have time right now.
By the way, if you don't know right words, then read books. Reading have positive influence on your "inner vocabulary".