It’s not as simple as comparing the TER of Viac and VT. Not having to pay taxes on dividends is huge. I just did some quick calculations on what having to pay taxes on your dividends in terms of TER means.
Assuming an investment of 6826 and a dividend yield of 2.5% (CHF 170) the following tax savings will apply for Viac when comparing to investments outside of 3a:
If you have 100k taxable income: (No church tax)
Freienbach (SZ): CHF 23 -> -TER 0.34%
Zürich (ZH)): CHF 33 -> -TER 0.48%
Les Verrières (NE): 44 -> -TER 0.65%
If you have 200k taxable income: (with church tax)
Freienbach (SZ): CHF 36 -> -TER 0.53%
Zürich (ZH)): CHF 56 -> -TER 0.82%
Les Verrières (NE): 62 -> -TER 0.81%
If you want to compare to VT than of you also have to take higher Kapitalzahlungssteuern into account that applies in 3a. Which will vary depending on where you live at the time when you will take out the money of your 3a and how much more capital will be in your 3a because you invested during the year and not just at the end of the year. Assuming you take out 25k at once in SZ this will be less than 1% of your additional capital gain more, in ZH and NE more like 3% more.
When comparing to VT than the dividend tax leakage on US Equities also has to be taken into account. Assuming a 35% allocation in S&P 500 in Viac and the same Dividend yield of 2.5% will result into a higher TER of 0.13% and a loss of a rounded up CHF 9.
Assuming I made the calculations correctly you can see that for some high income people in high taxes locations Viac might actually be more attractive. But for most people you are right and VT is better. VWRL is a different story. And if being close to the total market and not overweight in Switzerland is the most important thing to you than VT/VWRL are to be preferred anyway.