There are several problems
1.) Nothing Leaves Ravenloft
A central problem in Ravenloft is that it is all but impossible to leave that plane. Relevant text can be found here (Curse of Strahd, p. 24, bold added):
No spell - not even wish - allows one to escape from Strahd's domain. Astral projection, teleport, plane shift, and similar spells cast for the purpose of leaving Barovia simply fail, as do effects that banish a creature to another plane of existence. These restrictions apply to magic items and artifacts that have properties that transport or banish creatures to other planes.
As such, the bag of holding would not be able to transport a creature to another plane of existence.
2.) Heart Stabbing is not a given
If you could stab a creature in the heart whenever it was incapacitated automatically, then THL would be a save-or-die spell for most creatures. 5e doesn't have specific rules on "called shots", but there's some guidance given, with relevant text on hit points in the PHB p. 197:
When you drop below half your hit point maximum, you show signs of wear, such as cuts and bruises. An attack that reduces you to 0 hit points strikes you directly, leaving a bleeding injury or other trauma, or it simply knocks you unconscious.
In order to drive a wooden stake through Strahd's heart, you'd need to reduce him to zero hit points. Normally, this is done by attempting to stake the vampire when it is already at 0 hit points, which brings me to my next point.
3.) Timing Matters
Note the relevant text on staking a vampire (MM p. 297, bold added)
Stake to the Heart. If a piercing weapon made of wood
is driven into the vampire's heart while the vampire is incapacitated in its resting place, the vampire is paralyzed until the stake is removed.
It's not enough to have the vampire in its resting place, incapacitated, with a stake through its heart that was driven there earlier: you must drive the stake through its heart "while the vampire is incapacitated in its resting place." Unless Strahd was somehow in his coffin when you (successfully) cast tasha's hideous laughter on him, these terms would not be satisfied by your strategy.
4.) Spell Scrolls only work for certain people
Although any person can read a generic "magical scroll" to attempt to decypher its meaning, a spell scroll is a specific magic item with its own rules. Specifically (DMG, p.200):
If the spell is on your class's spell list, you can use an action to read the scroll and cast its spell without having to provide any of the spell's components. Otherwise, the scroll is unintelligible.
Since tasha's hideous laughter is a first level spell, I'm assuming that any character in your party who has it on their class's spell list could cast it already without a scroll. But anyone who doesn't have that spell on their class list will be unable to use the spell scroll to cast it.