Vel'Koz isn't very good at pushing. You beat him by standing out in front of his minions and zoning him from everything. You should have no trouble with this matchup as LeBlanc.
Ziggs is a good pusher. You have an advantage level 1 that you need to exploit to get faster level 2. Then you need to go all-in at level 2 to force him back or make a kill. After that, you zone him hard. If you don't take an early advantage here, your best bet is to clear the wave with your ult at level 6 and go roam for kills.
If you need to harass opponents that like to stand a long way away, you need to get good at WQE combinations -- W to about max Q range, Q and E them, W back when they try to fight back. This is pretty mana intense work, you're gonna want to buy early regen items.