The way I see it, we have four different alternatives:
Alternative 1a: Doc is Vampire 1 and he attacked Heathers.Vampire 1: Doc
-Turned someone into Vampire 2.
-Attacked Heathers.
-Died with 1/2 gallon of blood.
Vampire 2: Unknown
-Was turned by Doc.
-Is a man.
-Attacked Montana.
-Is Beardless.
-Was Gonzales' first attacker.
-Turned someone into vampire 3.
-May be Gonzales' killer.
-May have 1 gallon of blood.
Vampire 3: Unknown
-Is a man.
-May be Gonzales' killer.
-May have 1 gallon of blood.
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Alternative 1b: Doc is Vampire 1 and he attacked Montana.Vampire 1: Doc
-Turned someone into Vampire 2.
-Attacked Montana.
-Is Beardless
-Died with 1/2 gallon of blood.
Vampire 2: Unknown
-Was turned by Doc.
-Is a man.
-Attacked Heathers.
-Was Gonzales' first attacker.
-Turned someone into vampire 3.
-May be Gonzales' killer.
-May have 1 gallon of blood.
Vampire 3: Unknown
-Is a man.
-May be Gonzales' killer.
-May have 1 gallon of blood.
____________________________________________________
Alternative 2a: Doc is Vampire 2 and attacked Heathers.Vampire 1: Unknown
-Turned Doc into Vampire 2.
-Attacked Montana.
-Is beardless.
-Was Gonzales' first attacker.
-Turned someone into Vampire 3.
-May be Gonzales' killer.
-May have 1 gallon of blood.
Vampire 2: Doc
-Was turned by Vampire one.
-Is a man.
-Attacked Heathers.
-Lied about the other vampire being his heir.
-Died with 1/2 gallon of blood.
Vampire 3: Unknown
-Was turned by vampire one.
-Is a man.
-May be Gonzales' killer.
-May have 1 gallon of blood.
____________________________________________________
Alternative 2b: Doc is Vampire 2 and attacked Montana.Vampire 1: Unknown
-Turned Doc into Vampire 2.
-Attacked Heathers.
-Was Gonzales' first attacker.
-Turned someone into Vampire 3.
-May be Gonzales' killer.
-May have 1 gallon of blood.
Vampire 2: Doc
-Was turned by Vampire one.
-Is a man.
-Attacked Montana.
-Is Beardless.
-Lied about the other vampire being his heir.
-Died with 1/2 gallon of blood.
Vampire 3: Unknown
-Was turned by vampire one.
-Is a man.
-May be Gonzales' killer.
-May have 1 gallon of blood.
____________________________________________________
My opinion:I personally think alternative
1a is most likely, as I doubt the Doc would have made it back home in time to be seen walking in his door, although the vampire that bit Montana was faster than the other.
Also this:This could mean that this vampire's previous victim had screamed. Like Joe Montana: