I can understand where you're coming from, but if you're downloading from a trusted source (Such as vShare), then you have nothing to worry about.
Keeping unknown sources enabled isn't just for what you're downloading. Due to keeping that enabled, my phone was hacked. I kept it enabled for Amazon app store. Suddenly, my phone started giving me Japanese notifications on the bottom of the screen, one of which was network latency. My phone was tunneling somewhere and doing malicious things. I never manually installed anything other than Amazon, nor did I ever visit bad sites.
But yes, if getting from a reputable source, there should be nothing to worry about regarding the apk.