You'll need to make a temporary group or two to do it.
You'll copy the email from before and select the group that you had before (or select the recipients of the prior email).
You'll exclude the people in the groups that you make. Make the group(s) of the people who opened or already RSVP'd to your event first
Here's how to make the groups-
Step 1 - People that have opened the email
go to Search>Advanced Search
Select Mailings accordion.
Select your mailing
click the button for opened so that you get those who opened it.
Click Search take the option to add these people to a temporary regular group.
Step 2 - People that have RSVP'd
go to Search>Advanced Search
Select the Events Accordion
Select the event you are tracking
Select the appropriate participant status, if needed.
Click Search take the option to add these people to the group that you created above.
Step 3 - Sending the next Email
Now, when you prepare your next email, exclude the group you created from the mailing.
Optionally, these people can be put in smart groups, but you'll need to use two different ones.