First name adressing = same House? + Snape the Vampire (yes, again)

[Inge]

No: HPFGUIDX 84167

There was some talk recently that James and Lily might not have been 
in the same House at Hogwarts since they adressed each other by their 
last names.

I wonder in which house Hagrid was back then - do we have any canon 
to tell? 
The reason I wonder is because in CoS (chapter 13) Tom Riddle and 
Hagrid adress each other by first name:

"'Evening, Rubeus,' said Riddle sharply. The boy slammed the door 
shut and stood up. 'What yer doin' down here, Tom?'"

Riddle and Hagrid were 2 years apart when they were at Hogwarts and 
it seems a bit strange that they use first names (and this was in the 
40's when I assume that using last names was even more common between 
people who didn't socialise with each other?)

----

Second.
In PoA Sirius is at first described in a way that reminds me a lot of 
Snape (long, black hair, waxy white skin etc.) - and in Chapter 3 it 
says:
"Harry had never met a vampire, but he had seen pictures of them in 
his Defence Against the Dark Arts classes, and Black looked just like 
one."
I never thought of Black as a possible Vampire but as the describtion 
of him reminded me of Snape I (once again) got to think that there 
must be something there :-)

Inge