What can cause vessels in the arms to suddenly become visible? just noticed some vessels around my elbow show up more obvious. have been losing weight

Both skin&fat below: skin tend to make veins less visible, thus is loosing body fat, veins will become less hidden/more visible. Building muscles will increase vein prominence as will anything which increases pressure within veins which makes them distend (such as a tourniquet or clots downstream). Thus look for clues & be aware that greater visibility does not mean bad/unhealthy.