Whan i should ask for a rerol?

VAC bans can't affect whether the winner may be re-rolled or not, as it is an off-site matter. Other than that, if you notice something out of place such as not activated gifts, request a re-roll. Support will let you know if the user has already received a suspension for it or not.

• Non-activated gift - you can ask for a re-roll, but it depends on whether the person has already been suspended for it.
• Multiple wins of the same game - same as above.
• VAC ban - no, you cannot ask for a reroll, as a VAC has nothing to do with SG rules.
• Winner doesn't reply or activate within 7 days of the GA ending - you can ask for a re-roll.
• Winner declines the gift - you can ask for a re-roll.

Aside from what others already said, if you're doing private giveaways, you can always protect them with sgtools. It can for example check if someone has been VAC banned before giving them the giveaway link.

Seems like your question has been answered so have a random gif to start your week :D.

View attached image.

If your giveaway doesn't start with a SGTools gate, always check for for non-activated and multiple wins via SGTools. Install this script for to streamline the task (don't worry, it's easy to install and perfectly safe)

Always check for Non-Activations and Multi-wins, and use the Request New Giveaway Winner to report the issues if you find some. If staff notes the issues haven't yet been addressed, they'll approve of the reroll request. If not, they'll decline it and let you know why.
All you lose is a bit of time, and you end up making the site a better place.

(Staff does note not to use any other support ticket type, however, as they'll need you to make a Request New Giveaway Winner ticket either way before they can actually do the reroll, so it'll just waste time if you put in another ticket type first.)

I use this script to add SGTools links directly to user profiles, for fast profile checking.