I will be adding more grip paste, screws are already tightened to almost 5Nm and still slipping.

According to Chris, the longer screw in the front two hole limits how much you can tighten since it tops with the carbon as the hole is not that big. I'm not sure if it works, but I guess i have to try

I initially had the same problem with the handlebars slipping in the stem. I tightened them to 5.1 Nm, but that still didn't help with the pinched brake cables, which resulted in a crash and a complete cable replacement. I was lucky that the crash happened at very low speed. My mechanic initially complained about the poor quality of the stem – there wasn't enough clearance for proper clamping of the handlebars – the front jaws closed without any clearance. The problem was resolved after much persuasion, when Chris sent me a new stem, pre-tested and possibly modified. He also recommended lightly sanding the stem with sandpaper to increase clearance, if necessary. However, this wasn't necessary. The new stem securely locks the handlebars when tightened to 5 Nm with sanding paste. Therefore, I recommend not taking chances and requesting a new, tested stem, as pinched brake cables can cause a fall and injury.

i'm pretty sure of everything, but i don't understand why all the crankset are specified "BB wide 68/70mm", but the velobuild need a 86.5mm 

For Cransket 52/36 - shimano RS510 Shimano recomended BB-RS501 with Threaded bottom bracket shell width (mm) 68/70mm or BB-RS501-PB with Press-Fit bottom bracket shell width (mm) 86.5 mm. T47 86.5 will be compatible, IMHO.
Like this https://aliexpress.ru/item/1005005987688207.html A-T47-24(86.5MM)

Anyone having issues with their post slipping? I'm a smaller rider so I had to cut my seatpost a large amount. I had to cut off the bit of the seatpost that had the anti slip coating on it.

I haven't even ridden the bike yet and the post still slips just sitting on it... When I torque down the bolt as far as I can, the bolt bottoms out/slides against the clamping bit, and even then the post still has some side-to-side wobble like it's undersized.

I put a bit of electrical tape around the post and now I can at least sit on the bike without it slipping. I assume the post is a little bit undersized after I cut the bit off with the anti slip coating? Anyone know what the factories apply to the post that I can do at home?

Try that https://chinertown.com/index.php/topic,6206.msg96232.html#msg96232

Has anyone built a bike with an S(490) frame yet? Please share photos of the bike on the road and measurements!

@patliean1 in his video talks about the seatpost slipping despite very high tightening torque. I'll try to explain what could be the cause and how to fix it. The seatpost clamp is designed such that the part that presses against the seatpost has a limited forward extension. The opposite part of the clamp rests against the frame. If the seatpost seat is too large, the clamp won't be able to securely hold the seatpost when the bolt is fully tightened. This will be indicated by a high tightening torque and a large gap between the seatpost clamp and the seatpost or frame, depending on the clamp design. The VB-TT-023 frame uses the same seatpost and clamp, and everything works perfectly with a tightening torque of just 4.8 Nm for a rider weight of 73 kg. My TWITTER bike initially had large gaps, and they've gotten even larger over the past three years, causing the seatpost to slip. I added a soft metal spacer between the clamp and the frame (three layers of 1.4 mm each), and it's helping so far. I think this is a universal solution for any frame. However, VB should adjust their manufacturing process and better control the seatpost mounting space, IMHO.

AI suggested that this frame is most similar to the SIMPLON PRIDE frame https://www.simplon.com/en/Bikes/Roadbikes/Pride_b_292366

I have a big problem with the stem of my TT023.
It doesn't hold properly, so the basebar twists when there is a strong shock. For example, I have just ridden through a pothole and the basebar has twisted a little in the stem.

As a result, I can no longer brake as the brake cables are crushed.

I have already used carbon assembly paste and tightened all the screws evenly.

Any ideas or other experiences?

I have routed the brake lines from the brake levers to the brake bodies. I also tightened the stem at the bottom front first.

I had similar issues: the stem would spin on the steerer, and the handlebar would spin in the stem.
I ended up using WELDTITE paste, degreasing the bolts and seat threads, and tightening the bolts to 5.1 Nm crisscross on the handlebars and 5 Nm on the steerer. I contacted the manufacturer and they recommended tightening the bolts to 5.0 Nm-5.58 Nm. I constantly checked the bolts for tightness and tightened them to the target torque. So far, everything is stable.
I would recommend that you first contact the manufacturer with a request and follow their instructions.

Can you post photos of how you routed the hydraulic brake cables? My set of riderever triva brakes is still on its way from Japan and I am currently using cable-actuated brakes. And it is not a good experience so far. I have changed the cables twice and the braking power is still not satisfactory. I have routed the brake cables through the top holes of the stem and brought them inside the stock base bar holes on the lower side. It looks ugly and not so aerodynamic. These cable-actuated cables seem to be so hard to bend and go through the stem as hidden/internal routed cables. I am running 1x sram force with wireless blips so the only cables I have are the brake cables.

This is two brakes mecanical cables and one rear derauler mecanical cable.