I am a Vodafone customer, and I know why their service is so ****!
Vodafone use the "stable" profile - most UK FTTC ISP's use the "Gaming" profile. I didn't find this out till after switching and I've gone from 38Mb download sync speed to 33Mb. Not great.
Vodafone's supplied router is terrible. Why? Two reasons actually.
One the wifi is poor. They talk up AC standard and Beam Forming. Heads up Vodafone, it doesn't work. After switching I couldn't get wifi in my bathroom, ruining my "alone time".
Two, due to some firmware bug to do with FEC(?) errors, the router will re-sync the line to try and stablise it. Not good! The BT end sees all the re-syncs and thinks the line is unstable. The "stable" profile they use is more cautious about sync speeds and drops it down. The result, your sync speeds get lower and lower over the course of time. It settled at 25Mb for me. This is a line that was a solid 38Mb with Sky for 2 years. Hmm...should've eaten the price increase!
I managed to fix some of the above by buying a new router at my own expense. I won't plug makes/models, but suffice to say my wifi now reaches my bathroom again, and the sync speed has recovered to the above mentioned 33Mb. Still not 38Mb, but less of a noticeable reduction in day to day use.
Note that if you do change router like I did, it can take quite some time for sync speeds to recover. The BT end needs to see 7-9 days of stability before it tries to up your speed a little bit. In all I think it took about 2 weeks to get back up to 33Mb. My router reports my line is capable of the 38Mb I had previously, but the "Stable" or "Business" line profile Vodafone insist on using is preventing me from getting there.
Vodafone = AVOID! I will be leaving as soon as my contract ends. Wish I had known some of the above before joining, but a lot of the "real" informaiton about ISP's that customers need is hidden. Like the line profiles ISP's use. Not published or easy to find out.
P.S. If anyone cares...my cabling from master socket to router is CAT6. The cable the ISP sends you is garbage. Go on eBay and buy one, I guarrantee it will improve your FTTC sycn speeds immediately!