Rifle Scopes : Vortex Viper 4-12x40 PA Dead Hold BDC

Regardless, I know I likely wont ever get to do this again so I don't want to blow my chances of shooting a predator because my equipment is sub par.  I know my marksmanship will play a major roll, but I am constantly grappling in my mind whether or not my set up will be able to shoot accurately at a target 600+ yards out.  The longest ranges I have access to max out at 300yds.  I know my rifle is a good model (I know it's not custom) but my scope leaves me wondering...will a Vortex Viper 4-12x40 be sufficient or do I need to upgrade?  

I spent about $400 on this scope but keep seeing on forums and youtube hunting channels that the optics they use are 3-4 times the price I paid.   Is a Nightforce or Huskemaw scope that much better?  Like I said, I don't have a lot of money to lay down but at the same time do not want to blow this opportunity.  Does anyone have experience with the Vortex Viper?  Any help would be appreciated.

Thanks so much in advance and I look forward to your opinions and advice.

PS.  would having a custom turret made be a wise choice?  They only run $80-$180 from what I see.

Thanks again!

Prior experience in hunting has only been rabbit/varmint and bow hunting (nothing further than 100 yds out)....