I was just wondering if it's possible to take the basic Earth 1000 AD scenario that shipped with the game, and make the Vikings playable.

Open the wbs file(the map) with notepad and set playable civ =1 like here and maybe minorciv to zero:
(Could be you have to do the opposite for another civ)

BeginPlayer
Team=6
LeaderType=LEADER_WILLIAM
LeaderName=Justinian
CivDesc=Byzantine Empire
CivShortDesc=Byzantine Empire
CivAdjective=Byzantine
FlagDecal=Art/Interface/TeamColor/byzantine.dds
WhiteFlag=1
CivType=CIVILIZATION_DUTCH
Color=PLAYERCOLOR_ORANGE
ArtStyle=ARTSTYLE_EUROPEAN
PlayableCiv=1
MinorNationStatus=0
StartingGold=800
StateReligion=RELIGION_ISLAM
StartingEra=ERA_MEDIEVAL
CivicOption=CIVICOPTION_GOVERNMENT, Civic=CIVIC_FEUDAL_MONARCHY
CivicOption=CIVICOPTION_ECONOMY, Civic=CIVIC_FEUDAL_AGRICULTURE
CivicOption=CIVICOPTION_VALUE, Civic=CIVIC_SOCIAL_ORDER
CivicOption=CIVICOPTION_RIGHTS, Civic=CIVIC_ARISTOCRACY
CivicOption=CIVICOPTION_RELIGION, Civic=CIVIC_STATE_PROMOTION
AttitudePlayer=2, AttitudeExtra=7
AttitudePlayer=10, AttitudeExtra=9
AttitudePlayer=11, AttitudeExtra=-47
AttitudePlayer=13, AttitudeExtra=-98
AttitudePlayer=16, AttitudeExtra=-43
Handicap=HANDICAP_NOBLE
EndPlayer