Well the entire point of Vault 111 was to see the effects of extended cryogenic stasis on unaware/unsuspecting individuals. Vault-Tec nor the greater science community had a clue what would happen to the people, it's believable that you come out and your brain is so scrambled from being frozen for two hundred years that most of your original personality is rewritten as your brain tries to recover.
Roleplaying is fun
As for the pre-game introduction I agree... too short, it was very pretty and you understood the sense of urgency but I disliked having to learn about my preset character's military career via century old terminals. I think introducing new players to the combat aspects of the game in a flashback (before the Vault) of your character fighting in the trenches at the battle for Anchorage, Alaska would be nice... it'd help establish the history, and get you to like your main character... maybe put him through some scene where you're administering first aid to a wounded comrade and he's reaffirming to the dying soldier that he'll be okay as he bleeds out on the snow and mud.
He's portrayed as a kind, loving person who likes his peaceful life in Sanctuary Hills and has some knowledge and history in Boston's locales, but the only time I feel his attachment to the world before is when he chitchats with pre-war ghouls (who are very rare in Boston for some reason) when you activate the skeletons in the Vault and also when you inspect any cryopod and how his voice wavers and quivers a bit as he gets panicky that everyone is dead. I wish he/she made random comments as you explored the ruins of Boston, maybe you came across the veterans hall and the MC makes an offhand nostalgic comment about how if things had been different they might have been sitting there instead of standing in the ruins or something.
The voiced main character is simply under utilised.