In a data warehouse with small amount of writes, would RAID 1 be better than RAID 1+0? We are using HP Vertica database 7.2 community version.  300GB of data on one node. Thats a small amount of data.

HP officially recommends RAID 1+0. 
https://community.dev.hpe.com/t5/Vertica-Knowledge-Base/RAID-Storage-for-HPE-Vertica/ta-p/234225

But I do not envision a problem with write performance which is the point of RAID 0.  I am not planning to load, insert or write large amounts of data.  I am planning to query large amounts of data, but not write a lot.  It seems to me that RAID 1 is sufficient. 

Is there anything wrong with using RAID 1 instead of 1+0?
This is a pretty open ended question and you might want to just test it yourself and see.  It depends on how you query and write the data.  This will determine how many IOPS you need at peak use, which if you want to ensure you'd figure out by testing then deciding.  

However if that is not an option, do you have hundreds of gigs of RAM?  Some systems do.  If so, perhaps your read/write patterns will hit RAM and Disk IO won't matter.  Are you using SSDs or magnetic disks?  SSDs have a much higher disk IO and IOPS than traditional spinning media so the write penalty might not even be noticeable.

Really not sure how much more to add here, it'll come down to your testing results and IOPS needs.