There one thing you need to know about veno... he's extremely week. 4 or 5 hits from a melee hero and he's dead. So you need to float like a butterfly and sting like a bee. However in the early rounds veno don't float, he's as slow as a turtle smoking ganja. So he's my advice to you, get as many kills early in game, so that you can buy the stuff you need. Furthermore I think veno is one the most deadliest hero's early game, so it shouldn't be too hard to get kills.
Having the right stuff, makes this process easier, here's a list of what you'll need:
1) Lane with a melee hero with a stun or slow
2) Harass! harass! harass! soften up those hero's, use bottle w/flying courier, use gale alot to soften up, once bottle done give to courier to refill- saves on cost of buying tangos and clarities
3) Once you get a hero down to about 60-65% of their health, gank'em with gale, attack and have your partner attack do whatever it take to get the job done. If enemy lane partner kicking veno's *** then switch it up, attack him instead and let your partner kill of the other the hero then get out. (aka float like a butterfly sting like a bee)
4) Rinse repeat.
5) This strategy is not 100% of course but it works a lot if you have a smart lane partner. If not, your lane partner will get ganked, then you lose.
6) Important (find out if your lane partner is smart) also once the early kills start rolling in, someone will come gank you so watch out.
Use the money to adjust for what your team needs (aka support).