Definitely need to provide more details. I would highly recommend that you use aliases in your queries. Your table names are very long and not terribly user friendly. I took a shot in the dark. See if this is somewhat close to what you are looking for.

SELECT v.openamountusd AS OpenAmtUSD,

SUM(Case when v.duedate > getdate() then v.openamountusd else 0 end) as OpenPastDue,

s.siteid,

s.sitename,

s.division,

v.duedate,

ven.vendorname,

ven.vendorclass,

v.weekdue,

v.yeardue,

s.sitedescription

FROM vfact_aptransaction_datepart v

INNER JOIN dim_site s

ON v.siteid = s.siteid

INNER JOIN dim_vendor ven

ON v.vendorid = ven.vendorid

group by v.openamountusd,

s.siteid,

s.sitename,

s.division,

v.duedate,

ven.vendorname,

ven.vendorclass,

v.weekdue,

v.yeardue,

s.sitedescription