I'm not forced, I quite like working on Vim. However I develop on a multi-app setup, 3 backend services and 4 npm based frontend apps with watchers and works running in background. RubyMine/Webstorm has a hard time keeping up and the memory usage shoots up to several gigabytes. As for Slack, I counted just now - the compressed memory usage is ~750MB! These things seem insignificant but do add up, especially with Chrome which itself takes a few gigabytes.
I use an identical i5 model with Android Studio (think IntelliJ with more resource usage), an Android emulator/VM, a Gradle daemon taking up 2-3GBs for builds, Safari with umpteen tabs, Slack, Outlook and Spotify open and don't have issues with disk thrashing (or at least, none I can tell based on usage and the Activity Monitor memory pressure).
I don't know how you're forced to use Vim.
I've noticed that OSX will do what I'd like any modern OS to do and use the majority of my ram with a few things running things running. But adding more of a workload when it's using nearly 8GB doesn't instantly make it "run out of memory", more paging occurs but not nearly enough to cause disk thrashing (or be easily perceptible)