Branching vector addition systems are an extension of vector addition systems where new reachable vectors may be obtained by summing two reachable vectors and adding an integral vector from a fixed finite set. The reachability problem for them is shown hard for doubly-exponential space. For an alternative extension of vector addition systems, where reachable vectors may be combined by subtraction, most decision problems of interest are shown undecidable. Key words: program correctness, theory of computation