Well I don't think it's a fuchsia and I have a feeling that it's something that you don't want growing in your garden (having one of those moments when I know what it is but it's name just won't come to mind and I keep thinking of begonias - no idea why!!!)

Its pricking my mind too. any chance of a close up of a leaf and where it joins on the stem. are there small leaflets next to each main leaf?

Could it be Russian Vine, Fallopia baldschuanica?

would it be a root sucker from your grapes in the background

Don't think it's Russian Vine, I've grown that in the past. The stems are more mottled and the leaves a lot brighter green - plus it isn't known as mile-a-minute without cause.