Regarding "во Владивостоке" - is it "во" not "в" because the first letter of Владивостоке is "В"? And is the "во" pronounced unstressed or stressed?
Yes and also because the 2nd letter "л" is consonant so when trying to quickly say "в Владивостоке" the preposition may be lost. Compare "во Владивостоке" and "в Витебске".
And "во" is unstressed (almost always except may be in some obsolent expressions like in song "Во поле берёза стояла")