Well you are both right in a way.
Vilnius is today in Lithuania. But before ww2 it was in Poland and know unde rthe polish name of Wilno.
So back in 1925 it was definitlaly Poland. I'm not going to go into the whole historical debate ion wether Wilno is Polish or Lithuanian, as it has a complex history. Back in the 1920s the majority of the population was IIRc polish speaking catholics, but lithuanian-speaking catholics and various orthodox groups (including Belorussains) were a visible minority living in the city. There were also a jewish minority.
Its hard to be entirely sure, as the barriers between nationalities in the region were often somewhat fluid, but the surname of one of the perpetrators (Ławrynowicz) is imho definitally Belorussian. The other perp's surname (Obrąbalski) can be either polish or belorussian. One thing is sure, these are certainly neither jewish nor lithuanian surnames.
My best bet is that both shooters were Belorussians by ethnicity.
Check this thread, its been discussed in more lenght there:
[You must be registered and logged in to see this link.]
_________________
Life is like a tram - you need to know when to get off.
"Bullet Time" - a school shooting film from Poland