Can I ignore this error? I see it spammed in my console from time to time.

No. That should not be ignored. It usually means the vending machine is full. Vending machines can hold many items stacks, I forgot the exact number, but I think it's over 100. When the plugin is functioning properly, it should use at most 7 item slots because it's designed to bypass stack sizes and simply modify stack sizes of existing items when restocking. That was done because some server owners would configure stock so high that the vending machine would overflow, preventing some items from even being restocked when depleted.

If your vending machine is full, when a particular item is depleted, the supply signal item may be added first, preventing other items from being stocked.

We need to figure out why the supply signal isn't stacking properly and fix that. The way the plugin is designed, when restocking an item, it will find a matching item inside the vending machine that matches what it expects (item id, skin id, etc.), then increments that item's stack size. If the plugin does not find that item, it adds a new one. What is probably happening is that another plugin is altering the supply signal when it is added to the vending machine, causing Custom Vending Setup to no longer recognize the item. The Vehicle Airdrops plugin by Nikedemos is known to alter supply signals inside vending machines, by replacing them with a different item that has the same skin, to make clients render multiple skins correctly. Do you have that plugin installed? If so, is it up to date? (I vaguely recall we already fixed that but there may have been a regression or oversight if latest version.)

I use Heli Signals only and after last update I reconfigured it cuz there was config Issue. I used the old skinID but changed the Item name a little bit. Maybe this caused the issue... Vending is full with 2 different supply signals? Not sure. How I can fix this?

What might be happening is, if you changed the display name of the supply signal in the Helis Signals plugin, that plugin might have logic to automatically apply that display name to any supply signal with the configured skin. When you originally saved the vending machine, CVS would have recorded the display name, so it's probably creating the supply signal with the old name, then Heli Signals automatically changes the name, causing Custom Vending Setup to no longer recognize it. If that is the issue, try re-saving the vending machine, or removing the item from it and adding a new one.

OK good, I will try.