In comparing VWETX (LT corp) and VBTLX (bond mkt), I would suggest comparing yield, duration and credit quality. Vanguard has all this data on their website, on the overview and portfolio pages.
LT corp yields 2.6% ; Bond mkt yield of 1.9%.
LT corp has a 14.4 year duration ; Bond mkt a 6.2 year duration.
LT corp is 61% "A" rated bonds ; Bond mkt is 62% government bonds
I don't know how to turn credit quality into meaningful numbers, but investment grade bond categories are: gov't, AAA, AA, A, BBB. So Bond mkt has 62% highest quality bonds, while LT corp is one step up from the lowest quality of investment grade.
When bond yields go up, nobody wants an old bond - they can get paid more by purchasing at the new, higher rates. So the older bonds fall in price, and the duration expresses the amount. There is an inverse relationship, so you can use "-1" in calculations. Here's an example if bond yields moved up +0.25% :
LT corp has a 14.4 duration, so: -1 x 0.25 x 14.4 = -3.6%
Bond mkt has a 6.2 duration, so: -1 x 0.25 x 6.2 = -1.6%
So when is the 0.7% greater yield of LT corp overwhelmed by changes in bond yields?
I calculate if bond yields go up by 1/12th of 1%, LT corp and Bond mkt break even.
For 8 years of duration, you're only being paid 0.1% higher yield per year, which is next to nothing. You can also see the impact of credit quality in yesterday's market losses for these two funds.
LT corp -2.2% yesterday ; Bond mkt -0.60%
Considering the small difference in yield versus the higher duration and credit risk, I'd say Total Bond Market is the clear choice between the two.