Most people say that "dor" is one purely Romanian word as no decent translation has been proposed...
What about yearning or the German Sehnsucht? Or are you referring to the fact that dor is construed with an adverb (foarte) instead of an adjective?
"Yearn " would come close indeed. "Dor" is a noun and gets its full meaning when someone is"having" it, similar to thirst (sete) cold (frig), warm (cald), hungry (foame), well (bine), etc. ("Mie mi-e dor / Ție ți-e sete / Lui îi e frig / Nouă ne e cald / Vouă vă e foame / Lor le e bine" = I am missing (yearning/ longing for) / You are thirsty / He is cold / We are warm / You are hungry / They are well). Here ”foarte” is applied to the verb (”îmi este foarte...”) but you could also have ” Eu am un dor foarte mare” where ”foarte mare” is an adjective to the noun ”dor”. :-)
Is this correct: "Veți fi dor de noi foarte mult" ("You will miss us very much")?
No. "Vă va fi foarte (tare) dor de noi” would be correct. The 3rd person future tense of ”a fi” is ”va fi” ( present : ”este”)
As crbratu pointed out you need the third person because the construction is kind of impersonal: ”dor” is always the subject, so the verb does not change with the person(s) involved. You say ”îmi, ne, vă este dor” (I, we, you miss), and in the (literary) future tense this impersonal ”este” becomes ”va fi.”