Start your day, open the store. Most NPC's arrive earlier in the day which is how we will use this in our advantage.

Keep an eye out on the clock on the top left. Once it becomes mid-day, go ahead and force your store to close. You do this by interacting with the sign you use to open it. I close it around 14.00 +/-

From my experience the vomiter/npc's arrive around 10-12 most of the time.



Then repeat this until the vomiter arrives. Once the vomiter has arrived and you've hit them, go ahead and close your store as multiple will not arrive on the same day.



TLDR

• Open store
  
• Wait until 14.00 or until vomiter arrives
  
• Force close store
  
• Repeat

Good luck!