The only dictionary I have to consult regarding what a "book bag" is, suggests that a book bag is not the same thing as a backpack. The dictionary seems to agree with you though, that a "knapsack" IS much the same thing as a backpack... and I've added "rucksack" to the list of hope-to-be-accepted-in-the-future vocabulary.
However, "is" is definitely the wrong form of the verb "to be" to use in your sentence/question above. Adjusting your question to put it in sentence form (and adding the indefinite article "a" which your question ought also to have contained) gives:
"A book bag and knapsack is technically the same thing." This is wrong.
The subject is plural. There are two items - a book bag and a knapsack - and so the verb needs to be the plural form "are" not the singular form "is".
ie. Aren't a "book bag" and "knapsack" technically the same thing?
I'm not trying to have a go at you personally, but English speakers in general do a very poor job of choosing the plural form of the verb "to be" when it's required, especially in the constructions: "There is/are... ; There was/were... ; There has/have been ... ".
Some friendly advice: a more humble, helpful attitude might make you more friends.
What exactly is ba lô in Vietnamese? I'm afraid I don't really understand your answer.
Since there seems to be some confusion here, maybe I'd better err on the side of too-many-details. Everything I'm about to say is based on my experience on the East and West Coasts of the US.
Where I grew up (East Coast), children carried books and other belongings to school in a sturdy canvas bag worn on the back. We usually called the bag a "backpack." A few people called it a "bookbag." I believe "knapsack" and "rucksack" are less commonly used words for the same thing, but I don't recall hearing them as a child (except in books).
Nowadays young adults wear backpacks to work as well (at least on US East and West Coasts). I think in my parents' day this didn't happen as much: adult men preferred briefcases, and backpacks were mostly for children. But now it's very common. These adult backpacks are adult-sized but otherwise basically the same as a schoolchild's backpack.
Also, people who go hiking or camping carry backpacks for tents, food and so forth. These backpacks are sturdier and larger. I think they are designed so the weight is supported on the waist as well as the shoulders. Hikers need to carry a lot more weight, for much longer hours. By the way, hiking and camping are sometimes referred to as "backpacking": "I spent three days backpacking in the California wilderness."
"Backpack" is my default word for all these items. I've heard "bookbag" used for schoolchildren's backpacks only. I understand "knapsack, rucksack, satchel" are words for something similar but I've never heard these words in day-to-day life so I'm not sure if there are any subtleties of meaning.
Now, what are some words for these different kinds of backpacks in Vietnamese? Are they all called ba lô? Or, maybe Vietnamese backpack culture is different, and backpacks aren't used in the same contexts...? Maybe there's room for an interesting discussion here.
Or perhaps you'd rather lecture me on English subject-verb agreement. :)
I grew up in Australia and I've heard of backpack, satchel, knapsack and rucksack but only ever use and hear the word backpack used in common conversations. I'd only ever encounter such words as knapsack and satchel in high fantasy/medieval-themed MMORPGs. Also, ba lô is just a backpack.