Nikki is done regardless on Thursday.
Tim, Joel and Cass are planning to push Tim/Cass/Nikki as a threesome. Tim/Cass plan to hang out all week with Nikki to make it seem like they're a threesome. They said the brothers and Kelsey will put up Tim/Cass if they win the next HoH, but if one of them wins Veto, they want Nikki to go up instead of Joel so then the Veto winner and Joel can vote out Nikki and keep their three in tact.
Only way Nikki survives past Thursday is if Kels/brothers win HoH AND Veto, but even then, they may nominate Tim/Nikki over Tim/Cass.
Edit: Oh, I guess she also survives if Tim/Joel win HoH, too. Heck, she might even win it herself