“Flashback [cost]” means “You may cast this card from your graveyard by paying [cost] rather than paying its mana cost” and “If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack.”

If a card with flashback is put into your graveyard during your turn, you can cast it if it's legal to do so before any other player can take any actions.

If you leave the game while controlling permanents owned by other players that entered the battlefield under your control, those permanents are exiled. If the other players leave the game, they take their cards with them. (By contrast, if you leave the game while controlling permanents owned by another player you gained control of, those control-changing effects end and the permanents return to their previous controllers.)

You must still follow any timing restrictions and permissions, including those based on the card's type. For instance, you can cast a sorcery using flashback only when you could normally cast a sorcery.

The cost-reduction effect that applies if you cast Visions of Dread using flashback applies only to the generic portion of the cost to cast it, although it can apply to any additional costs there may be to cast the spell. It can't reduce any colored mana required to cast the spell.

You can cast a spell using flashback even if it was somehow put into your graveyard without having been cast.

If your commander is under the control of another player as you cast Visions of Dread using its flashback ability, that commander's mana value can be used to determine the value of X.

A spell cast using flashback will always be exiled afterward, whether it resolves, is countered, or leaves the stack in some other way.

To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.