Dim res1 as variant 'may return an error (like #n/a in the worksheet
dim res2 as variant
dim myStr as string

res1 = application.vlookup(xcombo.value, searchrange, 3, false)
res2 = application.vlookup(ycombo.value, searchrange, 5, false)

if iserror(res1) _
or iserror(res2) then
   mystr = "Some kind of error with vlookup"
elseif isnumeric(res1) _
    and isnumeric(res2) then
     mystr = format(res1 + res2, "#,###.00") 'or no format???
else
   mystr = "at least one non-numeric found"
end if

nonfincalcLabel.Caption = mystr

====
Untested, uncompiled.  Watch for typos.

Application.vlookup() returns an error that you can test with iserror().

application.worksheetfunction.vlookup() causes a runtime error that you have to
catch.

dim res as variant
on error resume next
res = application.worksheetfunction.vlookup(....)
if err.number <> 0 then
 'an error was found
 err.clear
else
 'no error!
end if
on error goto 0

application.vlookup() looks/works much easier (well, to me).