You can find the Affinity Photo vignette filter in the filters menu and colors category (destructive effect) and you can find it in the layers panel (non destructive effect)
The panel allows you to modify the exposure and setting it to the minimum for a black edge and setting it to the maximum for a white edge. The hardness can also be modified, set to 100% for a sharp edge and set to 50% or 0% for a blurry edge. You can also modify the scale for the border and set it low enough and the entire image is covered by the design, set it high enough and the border will not be seen. You can also modify the shape, set to 100% for a perfect circle and set closer to zero for an ellipse. It should be noted that the color effect needs to be applied to a layer and it needs to be a layer filled with an image, it does not add a vignette to an empty layer so you cannot move the origin of the effect which is centered on the center of the document
The effect can be applied to one layer or all the layers below. The live effect is non destructive and can be removed and modified at any point. You can also modify the blending mode of the effect such as creating an edge that is darkening or lightening the image. You can also move the color layer in the layers panel. You can also add adjustment layers on top of the color effect and then change the color of the border / edge effect.
You can also use the border effect with the fade command found in the layer menu and you can then change the opacity and blend of the applied border effect. If you don't want the effect to totally wipe out the edge of your image, the fade is an easy way to tone down the intensity of the effect.
You can also apply the effect multiple times by using the repeat option found at the top of the menu to intensify the border / edge effect such as making it solid black or solid white. You can apply others using this as well, great workaround for effects that go up to a certain limit
You can apply the effect to layers and that may require some modification of the scaling. It should be noted that the effect is still centered on the center of the document.
You can also use the effect with selections, any type of selection but the result works as if applied to the entire document but it is only generated inside the selection.
You can also apply the edge effect to channels so instead of using the RGB, red channel etc can be used with the effect. You can apply a different edge to green and so on. You can also create an alpha channel and add the effect to that.
The effect can also be used in conjunctions with shape layers (great for creating lunar moon effects) as well as text. All kinds of amazing color effects can be created using this powerful effect with four great options but it is even works super well with others such as the Deform and Motion and Mirror and many others
You can also use the effect with pattern layers, all can be used with the layers and they work even better when used with the mirror option. It does make it hard to see the effect as the part of the tile may not be in the range of the effect though some tweaking of scale will change that. If you use the live layer as the top level then the edge effect is applied properly to the entire image and all the tiles below which are all still live.
Sadly, for some reason considering all the other effects, is that there is no center of origin for this. It is always applied to the center of the document and not the center of the layer etc. One not particular ideal workaround is to use the crop feature where you can crop it larger than the document so you end up with your picture taking up only part (the top quarter or perhaps the middle of the bottom etc and then apply the effect to that and then you will get a different origin point (it will be to the center of the larger document). You can then re-size using the crop to put it back to the original size and now you will see the edge applied to a different origin.