> "Hvað færir þú til þess?" segir Höskuldur.

'You deliver (assign) what to that (what is your justification in
saying that)?,' says Höskuld.

I think that's right: what do you bring to that (by way of evidence,
reason, justification).

> vonfola (vánfoli ?)

Presumably so. I don't know the textual history of this word here, but
CV refers 'vánfoli' to 'váfoli', 'váfolald' "a vicious horse" [
http://lexicon.ff.cuni.cz/png/oi_cleasbyvigfusson/b0684.png ] - top of
left column. Only that seems to clash with his 'óreyndur' explanation.

> En þó að því sé að skipta og segir þú það satt að annaðhvort sé að

eg drepi þá eða þeir mig

> But even-though (it) were to turn-out like that (see under skipta,

Z5) and you say that truly, that either (it) be that I kill them or
they (kill) me,

At first I was reading it as: "Even if it turns out that you're
telling the truth, so that either [...]", but on second thoughts,
maybe it makes more sense as a prophesy: "Even if it does turn out
like that, that they do END UP plotting my death in the future (as a
result of your machinations), so that it will either be that I should
come to kill them or they me [...]"

> að verri

"the worse". This is covered in Zoega's third entry entitled 'at'
"demonstrative particle before a comparative". He translates this
example "so much the worse". Rather like 'því' before a comparative,
and 'en'/'in' [
http://lexicon.ff.cuni.cz/png/oi_cleasbyvigfusson/b0127.png ].