Firstly, I would change the booking with Alamo if you haven't paid anything yet. It gets far too many complaints. If you are not able to change, then demand a refund if they do not provide the car they promised at the price agreed. Check the car carefully for damage, working lights etc. and take photos if possible.

A car is not necessary in the city of Santiago and may complicate you more than help, unless you are doing excursions outside the city. There is a good metro system and taxis are not expensive.

Your rental car should come equipped with a TAG, an electronic device, which will beep under portals on the urban highways. On the road to the coast you will have tolls. The price is clearly marked by each booth. It is more expensive on weekends. You just hand the girl some money and she will give you your change and a receipt. You don't need to speak Spanish. The road is excellent. On the highway brown signs are informational and will indicate vineyards etc. If you pass something by mistake, look for RETORNO, a place you can double back.

Parking in Valparaíso is a very difficult, and in Viña to a lesser degree. Parking in either in February is a challenge. You may have to leave the car a fair way away. Be prepared to walk and don't leave anything in the car. There are a lot of one way streets, curves and hills in Valparaíso.

If you are staying at least a night in Valparaíso or Viña and want to stop at the vineyards then a car is useful going and returning. But the drink driving laws are much stricter in Chile than the US or Europe, so don't taste and drive.

One possibility is to get a private tour of the Casablanca valley with a drop off in Valparaíso and return on the bus.