30W's documentation has muddied the waters, however you cannot write down every last detail for every minor difference and common sense must be used. It is impossible to reissue a level restriction every time JUST because an aircraft has been put on a heading, if we go down that route, then how far do you want to go? Reissue it again and again every time the heading is changed? Vectoring is a method employed by ATC to provide separation from other aircraft, it is not a change of routing.
Ironically, the UK AIP only mentions that issuing a level clearance cancels any previous level restrictions unless the previous restrictions are repeated.