Good idea to move that vent. I don't see any problem either with the distance and 2 elbows.
Code requirement for OUTSIDE in my Municipality is 3'. This is because 'they' don't want snow to block the vent. Yes, too, there is a height restriction for the piping that comes directly OUT of the dryer. You might be best to ctc the MFGR. of the dryer to find out the MAX height the piping should be. 8' should be OK but just to be sure, check w/them. For all I know, you have a top-and-bottom washer/dryer setup vs. 2 stand-alone units.