How to architect an analytical BI platform?
With respect to your question about using Vertica during the ETL, it's very rarely necessary (although not unknown) to use a different type of database for the ETL.  I would not do that unless you perceive a specific need to do so.  The only times I've ever heard of this being done due to interactions with legacy data sources.  Although @Kermit works with Vertica and will be more familiar with the platform, I don't see any reason to assume you need another DBMS platform for your ETL.

The reason Vertica and other systems are ill suited to transactional applications is that they use data structures that are fast to read from and batch load, but have a significant overhead per load operation.  A batched ETL process that requires index rebuilds or other such operations will be a non-issue on these systems.  The operations are too expensive for a high-volume OLTP platform but will not be too slow for a batched ETL job.

A common architecture used in telcos and other VLDB shops is to use flat files for intermediate storage and then load into the database.  Ab Initio and Ascential Datastage are tools that are designed to work in this way and this type of architecture is common on telcos.  However, ETL tools tend not to support complex transformations well, so you may well end up with a layer of data manipulation taking place within the database.

All VLDB platforms are expensive, and some are tied to proprietary hardware.  Be prepared to spend up big on your server.  3 years data at 50GB/day is around 50TB, which is getting into the region where you would need specialised kit to perform well on these data volumes.