FO3 was just as long? the whole point is to do side quests, explore etc!
if you ran through FO3 like you clearly have done with vegas, you would have never of seen the alien crash site, the oasis, hidden vaults etc....whilst i havent played NV yet im sure their are many similar hidden gems that reward exploration and taking your time.
complaining about an open ended game being short is a bit silly as your kinda missing the point.
i remember plowing a good 50 hours into FO3 and was only near the end, infact i was so into exploring, side quests that i didnt even no what the main quest was about untill i got to the water chamber!