Should I continue increasing my Vcore? (i7-8700K @4.7GHz)

How well you can oc a 8700K is determined by your luck in getting a good chip.
as of 3/22/2018
What % of I7-8700k chips can oc
at a aggressive vcore near 1.4 or so and delidded
4.9 99%
5.0 88%
5.1 54%
5.2 22%

Note the delidded caveat which helps to keep temperatures under control.
Your NH-D15s is an outstanding cooler so you will have no problem with heat.

I think if you continue and get the vcore to 1.3 or so you will be fine.
1.4v is commonly suggested as an aggressive limit and I think the intel spec max is 1.5v.
I would not go near that.

While stress testing, monitor the vcore with cpu-Z.
You should also implement speedstep and adaptive voltage.
That will reduce the multiplier and vcore when the processor has little to do.

When you find your limit, back off a notch or two.
Really, how much do you need that last multiplier?

While it is true that each chip's overclocking potential is slightly differently due to manufacturing inconsistency and imperfection, this STILL doesn't seem right to me.

For a 4.7 ghz overclock on the i7 8700k, you put in 1.3V for Vcore and it's still not stable within a relatively short prime test? Hm.. I suspect there must be something else going on here.

Typically all 8700k should be able to hit 5.0 ghz easily for sure
... unless you get a super ultra bad chip

mysterry,

The difference between versions involves a huge variable called "AVX" (Advanced Vector Extensions), which has a profound effect on Core temperatures. Version 26.6 is non-AVX, but later versions run AVX, which imposes the most severe processor overload at nearly 130%. This workload is unrealistic and is exceedingly brutal far beyond any other utility or real-world application.

Although "Throttle" temperature (Tj Max) for the 8700K is 100°C, we know that cooler is better for ultimate stability, performance and longevity. As such, it's prudent to observe a reasonable thermal margin below Tj Max. Here's the nominal operating range for Core temperature:

Core temperatures above 85°C are not recommended.

Core temperatures below 80°C are ideal.

Core temperatures are directly affected by ambient temperature, which is important to take into consideration. Unless you provide a constant year-around "normal" or "standard" indoor ambient temperature (22°C or 72°F), seasonal temperature fluctuations can cause exceedingly high Core temperatures during summer months.

Not all loads are created equal. “Stress” tests vary widely and can be characterized into two categories; stability tests which are fluctuating workloads, and thermal tests which are steady workloads. Intel tests their processors at a steady 100% TDP workload to validate Thermal Specifications.

Prime95 version 26.6 Small FFT's is ideal for CPU thermal testing, because it's a steady 100% workload with steady Core temperatures that typically runs Core i variants with Hyperthreading (8700K) within +/- a few % of TDP. No other utility so closely replicates Intel's workload test conditions.

Utilities that don't overload or underload your processor will give you a valid thermal baseline. Here’s a comparison of utilities grouped as thermal and stability tests according to % of TDP, averaged across six processor Generations at stock settings rounded to the nearest 5%:

All tests will show 100% CPU Utilization in Windows Task Manager, which indicates processor resource activity, not % TDP workload. Core temperatures respond directly to Power dissipation (Watts), which is driven by workload. Prime95 v26.6 Small FFT’s provides a true and steady 100% workload, so if Core temperatures are below 85°C, then your processor should run the most demanding real-world workloads without overheating.

4th through 9th Generation i9, i7, i5 and i3 CPU’s have AVX Instruction Sets. Prime95 versions later than 26.6 run AVX code on the CPU's Floating Point Unit (FPU), which is an unrealistic workload that can cause Core temperatures up to 20°C higher.

Many 6th through 9th Generation motherboards address the AVX problem by providing “offset” adjustments (downclock) in BIOS. -3 (300 MHz) or more may be needed to limit Core temperatures to 85°C. If you don’t use AVX apps such as for rendering or transcoding, then you don't need to be AVX stable just to test thermal performance. However, BIOS should still be configured for it, as certain utilities use AVX for stability testing.

AVX can be disabled in Prime95 versions later than 26.6 by inserting "CpuSupportsAVX=0" into the "local.txt" file in Prime95's folder. However, since Core temperatures will be the same as 26.6, it's easier to just use 26.6. As per Intel’s Datasheets, TDP and Thermal Specifications are validated “without AVX”.

If you'd like to get yourself up to speed on this topic, then you need to read: Intel Temperature Guide - http://www.tomshardware.com/forum/id-1800828/intel-temperature-guide.html

CT