Q1: What does a "can't trash stacked cards" effect do, exactly?
This effect prevents cards stacked on top from being trashed by De-Digivolve or other effects, and cards stacked on the bottom can't be trashed by effects such as those that trash digivolution cards.
Q2: Can I return just 1 Vemmon to the bottom of the deck for the conditions of this card's inherited effect?
No, you can't. A "by doing X" condition can't be met if only some of the required actions are performed. The conditions for this inherited effect require you to return the specified number of cards to the bottom of the deck.
Lv.5 Destromon BT21-060 U Ultimate | Unknown | Unknown/LIBERATOR [Digivolve] [Vemmon]: Cost 6 [When Digivolving] Until your opponent's turn ends, their effects can't trash this Digimon's stacked cards. Then, to 1 of your opponent's Digimon, <De-Digivolve 1> for every 2 [Vemmon] in this Digimon's digivolution cards. [All Turns] When this Digimon would leave the battle area, you may play 1 [Vemmon] from its digivolution cards without paying the cost. --- Inherited: [Opponent's Turn] [Once Per Turn] When one of your opponent's Digimon attacks, by returning 2 [Vemmon] from this Digimon's digivolution cards to the bottom of the deck, end that attack.
Lv.5 Destromon BT21-060 U Ultimate | Unknown | Unknown/LIBERATOR [Digivolve] [Vemmon]: Cost 6 [When Digivolving] Until your opponent's turn ends, their effects can't trash this Digimon's stacked cards. Then, to 1 of your opponent's Digimon, <De-Digivolve 1> for every 2 [Vemmon] in this Digimon's digivolution cards. [All Turns] When this Digimon would leave the battle area, you may play 1 [Vemmon] from its digivolution cards without paying the cost. --- Inherited: [Opponent's Turn] [Once Per Turn] When one of your opponent's Digimon attacks, by returning 2 [Vemmon] from this Digimon's digivolution cards to the bottom of the deck, end that attack.
Lv.5 Destromon BT21-060 U Ultimate | Unknown | Unknown/LIBERATOR [Digivolve] [Vemmon]: Cost 6 [When Digivolving] Until your opponent's turn ends, their effects can't trash this Digimon's stacked cards. Then, to 1 of your opponent's Digimon, <De-Digivolve 1> for every 2 [Vemmon] in this Digimon's digivolution cards. [All Turns] When this Digimon would leave the battle area, you may play 1 [Vemmon] from its digivolution cards without paying the cost. --- Inherited: [Opponent's Turn] {{OncePerTurn}} When one of your opponent's Digimon attacks, by returning 2 [Vemmon] from this Digimon's digivolution cards to the bottom of the deck, end that attack.
Lv.5 デストロモン BT21-060 U 完全体 | 不明 | 不明/リベレイター 【進化時】相手のターン終了まで、このデジモンに重ねられているカードは相手の効果で破棄されない。その後、相手のデジモン1体をこのデジモンの進化元の「ベムモン」2枚ごとに、≪退化1≫。 【お互いのターン】このデジモンがバトルエリアを離れるとき、このデジモンの進化元から、「ベムモン」1枚をコストを支払わずに登場できる。 --- Inherited: 【相手のターン】[ターンに1回]相手のデジモンがアタックしたとき、このデジモンの進化元から、「ベムモン」2枚をデッキの下に戻すことで、そのアタックを終了する。