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No. It's not a spell, it's not an activated ability, it's not a triggered ability. Replacement effects don't use the stack.
405. Stack
405.1. When a spell is cast, the physical card is put on the stack (see rule 601.2a). When an ability is activated or triggers, it goes on top of the stack without any card associated with it (see rules 602.2a and 603.3).
405.2. The stack keeps track of the order that spells and/or abilities were added to it. Each time an object is put on the stack, it's put on top of all objects already there.
405.6. Some things that happen during the game don't use the stack.
405.6a Effects don't go on the stack; they're the result of spells and abilities resolving. Effects may create delayed triggered abilities, however, and these may go on the stack when they trigger (see rule 603.7).
No, "If Lotus Vale would enter the battlefield, sacrifice two untapped lands instead. If you do, put Lotus Vale onto the battlefield. If you don't, put it into its owner's graveyard." is neither an activated ability nor a triggered ability, so it can not be the target of Stifle.
602. Activating Activated Abilities
602.1. Activated abilities have a cost and an effect. They are written as "[Cost]: [Effect.] [Activation instructions (if any).]"
603. Handling Triggered Abilities
603.1. Triggered abilities have a trigger condition and an effect. They are written as "[When/Whenever/At] [trigger condition or event], [effect]. [Instructions (if any).]"
"When Lotus Field enters the battlefield, sacrifice two lands." is a triggered ability and can be countered by Stifle.ISBPathfinder wrote: ↑3 years agoLands don't normally use the stack so I was left a bit unsure if a stifle effect could work with Lotus Vale where as I understand it does with Lotus Field being an ETB after the fact effect.